CD8+ cytotoxic T lymphocytes (CTLs) are critical for clearing many viral infections, and protective CTL memory can be induced by vaccination with attenuated viruses and vectors. Non-replicating vaccines are typically potentiated by the addition of adjuvants that enhance humoral responses, however few are capable of generating CTL responses. Adjuplex is a carbomer-lecithin-based adjuvant demonstrated to elicit robust humoral immunity to non-replicating antigens. We report that mice immunized with non-replicating Adjuplex-adjuvanted vaccines generated robust antigen-specific CTL responses. Vaccination by the subcutaneous or the intranasal route stimulated systemic and mucosal CTL memory respectively. However, only CTL memory induced by intranasal vaccination was protective against influenza viral challenge, and correlated with an enhancement of memory CTLs in the airways and CD103+ CD69+ CXCR3+ resident memory-like CTLs in the lungs. Mechanistically, Myd88-deficient mice mounted primary CTL responses to Adjuplex vaccines that were similar in magnitude to wild-type mice, but exhibited altered differentiation of effector cell subsets. Immune potentiating effects of Adjuplex entailed alterations in the frequency of antigen-presenting-cell subsets in vaccine draining lymph nodes, and in the lungs and airways following intranasal vaccination. Further, Adjuplex enhanced the ability of dendritic cells to promote antigen-induced proliferation of naïve CD8 T cells by modulating antigen uptake, its intracellular localization, and rate of processing. Taken together, we have identified an adjuvant that elicits both systemic and mucosal CTL memory to non-replicating antigens, and engenders protective CTL-based heterosubtypic immunity to influenza A virus in the respiratory tract. Further, findings presented in this manuscript have provided key insights into the mechanisms and factors that govern the induction and programming of systemic and protective memory CTLs in the respiratory tract.
Current respiratory-virus vaccines typically employ non-replicating antigens and rely solely on the generation of humoral responses for protection. Viruses such as influenza can mutate and escape these responses, thereby limiting immunity and necessitating revaccination. Cell-mediated immunity (CMI) could provide broader protection by targeting viral components that infrequently mutate, however non-replicating vaccines capable of inducing CMI are not available. Impediments to vaccine development include an incomplete understanding of the nature of protective respiratory CMI and a lack of vaccine adjuvants capable of eliciting CMI to non-replicating antigens. Using a mouse model, we characterized the protective immunity afforded by CMI responses to non-replicating vaccines formulated with the adjuvant Adjuplex. We found that vaccination via either the subcutaneous or intranasal route was capable of inducing potent CMI responses. However, only intranasal vaccination protected against challenge with heterosubtypic influenza viruses. This protection correlated with enhancement of T cells with a resident-memory phenotype in the lungs. Additionally, mechanistic studies showed that Adjuplex affects antigen-presenting cells via activation and alteration of antigen uptake, processing, and presentation. The current studies: (1) identified an adjuvant that elicits protective CMI to respiratory viral pathogens; (2) suggested that stimulation of protective CMI in the respiratory tract requires intranasal vaccine delivery.
Background. Secondary bacterial infections after influenza can be a serious problem, especially in young children and the elderly, yet the efficacy of current vaccines is limited. Earlier work demonstrated that a replication-incompetent PB2-knockout (PB2-KO) influenza virus possessing a foreign gene in the coding region of its PB2 segment can serve as a platform for a bivalent vaccine.
Methods. In the current study, we generated the PB2-KO virus expressing pneumococcal surface protein A (PspA), PB2-KO-PspA virus, the replication of which is restricted to PB2-expressing cells. We then examined the protective efficacy of intranasal immunization with this virus as a bivalent vaccine in a mouse model.
Results. High levels of influenza virus–specific and PspA-specific antibodies were induced in the serum and airways of immunized mice. The intranasally immunized mice were protected from lethal doses of influenza virus or Streptococcus pneumoniae. These mice were also completely protected from secondary pneumococcal pneumonia after influenza virus infection.
Conclusions. These findings indicate that our recombinant influenza virus serves as a novel and powerful bivalent vaccine against primary and secondary pneumococcal pneumonia as well as influenza.
secondary pneumococcal pneumonia; replication-incompetent influenza virus; bivalent vaccine
Influenza viruses mutate frequently, necessitating constant updates of vaccine viruses. To establish experimental approaches that may complement the current vaccine strain selection process, we selected antigenic variants from human H1N1 and H3N2 influenza virus libraries possessing random mutations in the globular head of the haemagglutinin protein (which includes the antigenic sites) by incubating them with human and/or ferret convalescent sera to human H1N1 and H3N2 viruses. Further, we selected antigenic escape variants from human viruses treated with convalescent sera and from mice that had been previously immunized against human influenza viruses. Our pilot studies with past influenza viruses identified escape mutants that were antigenically similar to variants that emerged in nature, establishing the feasibility of our approach. Our studies with contemporary human influenza viruses identified escape mutants before they caused an epidemic in 2014–2015. This approach may aid in the prediction of potential antigenic escape variants and the selection of future vaccine candidates before they become widespread in nature.
The current outbreak of Ebola virus (EBOV) infection in West Africa is unprecedented, with nearly 26 000 confirmed cases and >10 000 deaths. Comprehensive data on the pathogenesis of EBOV infection are lacking; however, recent studies suggested that fatal EBOV infections are characterized by dysregulation of the innate immune response and a subsequent cytokine storm. Specifically, several studies suggested that hypersecretion of interleukin 1 receptor antagonist (IL-1Ra) correlates with lethal EBOV infections. To examine the significance of IL-1Ra in EBOV infections, we infected mice that lack the gene encoding IL-1Ra, Il1rn (IL-1RN-KO), and mice with wild-type Il1rn (IL-1RN-WT) with a mouse-adapted EBOV (MA-EBOV). Infected IL-1RN-KO mice lost more weight and had a lower survival rate than IL-1RN-WT mice infected with MA-EBOV. In addition, IL-1RN-KO mice infected with wild-type EBOV, which does not cause lethal infection in adult immunocompetent mice, such as C57BL/6 mice, experienced greater weight loss than IL-1RN-WT mice infected with wild-type EBOV. Further studies revealed that the levels of 6 cytokines in spleens—IL-1α, IL-1β, interleukin 12p40, interleukin 17, granulocyte colony-stimulating factor, and regulated on activation, normal T-cell expressed and secreted—were significantly different between IL-1RN-KO mice and IL-1RN-WT mice infected with MA-EBOV. Collectively, our data suggest that IL-1Ra may have a protective effect upon EBOV infection, likely by damping an overactive proinflammatory immune response.
Ebola virus; interleukin 1 receptor antagonist; Il1rn; IL-1Ra
Influenza A viruses are known to primarily replicate in duck intestine following infection via the oral route, but the specific role of neuraminidase (NA) for the intestinal tropism of influenza A viruses has been unclear. A reassortant virus (Dk78/Eng62N2) did not propagate in ducks infected via the oral route. To generate variant viruses that grow well in ducks via the oral route, we isolated viruses that effectively replicate in intestinal mucosal cells by passaging Dk78/Eng62N2 in duck via rectal-route infection. This procedure led to the isolation of a variant virus from the duck intestine. This virus was propagated using embryonated chicken eggs and inoculated into a duck via the oral route, which led to the isolation of Dk-rec6 from the duck intestine. Experimental infections with mutant viruses generated by using reverse genetics indicated that the paired mutation of residues 356 and 431 in NA was necessary for the viral replication in duck intestine. The NA assay revealed that the activity of Dk78/Eng62N2 almost disappeared after pH 3 treatment, whereas that of Dk-rec6 was maintained. Furthermore, to identify the amino acid residues associated with the low-pH resistance, we measured the activities of mutant NA proteins transiently expressed in 293 cells after pH 3 treatment. All mutant NA proteins that possessed proline at position 431 showed higher activities than NA proteins that possessed glutamine at this position. These findings indicate that the low-pH resistance of NA plays an important role in the ability of influenza A virus to replicate in duck intestine.
IMPORTANCE Neuraminidase (NA) activity facilitates the release of viruses from cells and, as such, is important for the replicative efficiency of influenza A virus. Ducks are believed to serve as the principal natural reservoir for influenza A virus; however, the key properties of NA for viral infection in duck are not well understood. In this study, we identify amino acid residues in NA that contribute to viral replication in ducks via the natural route of infection and demonstrate that maintenance of NA activity under low-pH conditions is associated with the biological properties of the virus. These findings provide insights into the mechanisms of replication of influenza A virus in ducks.
The genomes of influenza A and B viruses comprise eight segmented, single-stranded, negative-sense viral RNAs (vRNAs). Although segmentation of the virus genome complicates the packaging of infectious progeny into virions, it provides an evolutionary benefit in that it allows viruses to exchange vRNAs with other strains. Influenza A viruses are believed to package their eight different vRNAs in a specific manner. However, several studies have shown that many viruses are noninfectious and fail to package at least one vRNA. Therefore, the genome-packaging mechanism is not fully understood. In this study, we used electron microscopy to count the number of ribonucleoproteins (RNPs) inside the virions of different influenza A and B virus strains. All eight strains examined displayed eight RNPs arranged in a “7+1” configuration in which a central RNP was surrounded by seven RNPs. Three-dimensional analysis of the virions showed that at least 80% of the virions packaged all eight RNPs; however, some virions packaged only five to seven RNPs, with the exact proportion depending on the strain examined. These results directly demonstrate that most viruses package eight RNPs, but some do indeed package fewer. Our findings support the selective genome-packaging model and demonstrate the variability in the number of RNPs incorporated by virions, suggesting that the genome-packaging mechanism of influenza viruses is more flexible than previously thought.
The genomes of influenza A and B viruses contain segmented RNAs, which complicates genome packaging but provides the evolutionary advantage of allowing the exchange of individual genome segments with those of other strains. Some studies have shown that influenza A viruses package all eight genome segments in a specific manner, whereas others have shown that many virions are noninfectious and fail to package at least one genome segment. However, such viruses have never been directly observed. Here, we used electron microscopy to provide the first direct visual evidence of virions packaging an incomplete set of ribonucleoproteins. The percentage of these noninfectious virions varied from 0 to 20, depending on the virus strain, indicating that most virions package all eight genome segments. These results extend our knowledge about how infectious and noninfectious virions coordinate for successful virus infection.
New strategies to develop novel broad-spectrum antiviral drugs against influenza virus infections are needed due to the emergence of antigenic variants and drug-resistant viruses. Here, we evaluated C646, a novel p300/CREB-binding protein-specific inhibitor of histone acetyltransferase (HAT), as an anti-influenza virus agent in vitro and in vivo and explored how C646 affects the viral life cycle and host response. Our studies highlight the value of targeting HAT activity for anti-influenza drug development.
Highly pathogenic avian influenza viruses of the H5N1 subtype continue to circulate in poultry in Asia, Africa, and the Middle East. Recently, outbreaks of novel reassortant H5 viruses have also occurred in North America. Although the number of human infections with highly pathogenic H5N1 influenza viruses continues to rise, these viruses remain unable to efficiently transmit between humans. However, we and others have identified H5 viruses capable of respiratory droplet transmission in ferrets. Two experimentally introduced mutations in the viral hemagglutinin (HA) receptor-binding domain conferred binding to human-type receptors but reduced HA stability. Compensatory mutations in HA (acquired during virus replication in ferrets) were essential to restore HA stability. These stabilizing mutations in HA also affected the pH at which HA undergoes an irreversible switch to its fusogenic form in host endosomes, a crucial step for virus infectivity. To identify additional stabilizing mutations in an H5 HA, we subjected a virus library possessing random mutations in the ectodomain of an H5 HA (altered to bind human-type receptors) to three rounds of treatment at 50°C. We isolated several mutants that maintained their human-type receptor-binding preference but acquired an appreciable increase in heat stability and underwent membrane fusion at a lower pH; collectively, these properties may aid H5 virus respiratory droplet transmission in mammals.
IMPORTANCE We have identified mutations in HA that increase its heat stability and affect the pH that triggers an irreversible conformational change (a prerequisite for virus infectivity). These mutations were identified in the genetic background of an H5 HA protein that was mutated to bind to human cells. The ability to bind to human-type receptors, together with physical stability and an altered pH threshold for HA conformational change, may facilitate avian influenza virus transmission via respiratory droplets in mammals.
Arbidol (ARB) is a synthetic antiviral originally developed to combat influenza viruses. ARB is currently used clinically in several countries but not in North America. We have previously shown that ARB inhibits in vitro hepatitis C virus (HCV) by blocking HCV entry and replication. In this report, we expand the list of viruses that are inhibited by ARB and demonstrate that ARB suppresses in vitro infection of mammalian cells with Ebola virus (EBOV), Tacaribe arenavirus, and human herpesvirus 8 (HHV-8). We also confirm suppression of hepatitis B virus and poliovirus by ARB. ARB inhibited EBOV Zaire Kikwit infection when added before or at the same time as virus infection and was less effective when added 24 h after EBOV infection. Experiments with recombinant vesicular stomatitis virus (VSV) expressing the EBOV Zaire glycoprotein showed that infection was inhibited by ARB at early stages, most likely at the level of viral entry into host cells. ARB inhibited HHV-8 replication to a similar degree as cidofovir. Our data broaden the spectrum of antiviral efficacy of ARB to include globally prevalent viruses that cause significant morbidity and mortality.
IMPORTANCE There are many globally prevalent viruses for which there are no licensed vaccines or antiviral medicines. Some of these viruses, such as Ebola virus or members of the arenavirus family, rapidly cause severe hemorrhagic diseases that can be fatal. Other viruses, such as hepatitis B virus or human herpesvirus 8 (HHV-8), establish persistent infections that cause chronic illnesses, including cancer. Thus, finding an affordable, effective, and safe drug that blocks many viruses remains an unmet medical need. The antiviral drug arbidol (ARB), already in clinical use in several countries as an anti-influenza treatment, has been previously shown to suppress the growth of many viruses. In this report, we expand the list of viruses that are blocked by ARB in a laboratory setting to include Ebola virus, Tacaribe arenavirus, and HHV-8, and we propose ARB as a broad-spectrum antiviral drug that may be useful against hemorrhagic viruses.
The 1918-1919 influenza pandemic remains the single greatest infectious disease outbreak in the past century. Mouse and nonhuman primate infection models have shown that the 1918 virus induces overly aggressive innate and proinflammatory responses. To understand the response to viral infection and the role of individual 1918 genes on the host response to the 1918 virus, we examined reassortant avian viruses nearly identical to the pandemic 1918 virus (1918-like avian virus) carrying either the 1918 hemagglutinin (HA) or PB2 gene. In mice, both genes enhanced 1918-like avian virus replication, but only the mammalian host adaptation of the 1918-like avian virus through reassortment of the 1918 PB2 led to increased lethality. Through the combination of viral genetics and host transcriptional profiling, we provide a multidimensional view of the molecular mechanisms by which the 1918 PB2 gene drives viral pathogenicity. We demonstrate that 1918 PB2 enhances immune and inflammatory responses concomitant with increased cellular infiltration in the lung. We also show for the first time, that 1918 PB2 expression results in the repression of both canonical and noncanonical Wnt signaling pathways, which are crucial for inflammation-mediated lung regeneration and repair. Finally, we utilize regulatory enrichment and network analysis to define the molecular regulators of inflammation, epithelial regeneration, and lung immunopathology that are dysregulated during influenza virus infection. Taken together, our data suggest that while both HA and PB2 are important for viral replication, only 1918 PB2 exacerbates lung damage in mice infected with a reassortant 1918-like avian virus.
IMPORTANCE As viral pathogenesis is determined in part by the host response, understanding the key host molecular driver(s) of virus-mediated disease, in relation to individual viral genes, is a promising approach to host-oriented drug efforts in preventing disease. Previous studies have demonstrated the importance of host adaptive genes, HA and PB2, in mediating disease although the mechanisms by which they do so are still poorly understood. Here, we combine viral genetics and host transcriptional profiling to show that although both 1918 HA and 1918 PB2 are important mediators of efficient viral replication, only 1918 PB2 impacts the pathogenicity of an avian influenza virus sharing high homology to the 1918 pandemic influenza virus. We demonstrate that 1918 PB2 enhances deleterious inflammatory responses and the inhibition of regeneration and repair functions coordinated by Wnt signaling in the lungs of infected mice, thereby promoting virus-associated disease.
Marburg virus (MARV), a member of the filovirus family, causes severe hemorrhagic fever with up to 90% lethality. MARV matrix protein VP40 is essential for assembly and release of newly copied viruses and also suppresses immune signaling in the infected cell. Here we report the crystal structure of MARV VP40. We found that MARV VP40 forms a dimer in solution, mediated by N-terminal domains, and that formation of this dimer is essential for budding of virus-like particles. We also found the N-terminal domain to be necessary and sufficient for immune antagonism. The C-terminal domains of MARV VP40 are dispensable for immunosuppression but are required for virus assembly. The C-terminal domains are only 16% identical to those of Ebola virus, differ in structure from those of Ebola virus, and form a distinct broad and flat cationic surface that likely interacts with the cell membrane during virus assembly.
IMPORTANCE Marburg virus, a cousin of Ebola virus, causes severe hemorrhagic fever, with up to 90% lethality seen in recent outbreaks. Molecular structures and visual images of the proteins of Marburg virus are essential for the development of antiviral drugs. One key protein in the Marburg virus life cycle is VP40, which both assembles the virus and suppresses the immune system. Here we provide the molecular structure of Marburg virus VP40, illustrate differences from VP40 of Ebola virus, and reveal surfaces by which Marburg VP40 assembles progeny and suppresses immune function.
Mammalian host response to pathogenic infections is controlled by a complex regulatory network connecting regulatory proteins such as transcription factors and signaling proteins to target genes. An important challenge in infectious disease research is to understand molecular similarities and differences in mammalian host response to diverse sets of pathogens. Recently, systems biology studies have produced rich collections of omic profiles measuring host response to infectious agents such as influenza viruses at multiple levels. To gain a comprehensive understanding of the regulatory network driving host response to multiple infectious agents, we integrated host transcriptomes and proteomes using a network-based approach. Our approach combines expression-based regulatory network inference, structured-sparsity based regression, and network information flow to infer putative physical regulatory programs for expression modules. We applied our approach to identify regulatory networks, modules and subnetworks that drive host response to multiple influenza infections. The inferred regulatory network and modules are significantly enriched for known pathways of immune response and implicate apoptosis, splicing, and interferon signaling processes in the differential response of viral infections of different pathogenicities. We used the learned network to prioritize regulators and study virus and time-point specific networks. RNAi-based knockdown of predicted regulators had significant impact on viral replication and include several previously unknown regulators. Taken together, our integrated analysis identified novel module level patterns that capture strain and pathogenicity-specific patterns of expression and helped identify important regulators of host response to influenza infection.
An important challenge in infectious disease research is to understand how the human immune system responds to different types of pathogenic infections. An important component of mounting proper response is the transcriptional regulatory network that specifies the context-specific gene expression program in the host cell. However, our understanding of this regulatory network and how it drives context-specific transcriptional programs is incomplete. To address this gap, we performed a network-based analysis of host response to influenza viruses that integrated high-throughput mRNA- and protein measurements and protein-protein interaction networks to identify virus and pathogenicity-specific modules and their upstream physical regulatory programs. We inferred regulatory networks for human cell line and mouse host systems, which recapitulated several known regulators and pathways of the immune response and viral life cycle. We used the networks to study time point and strain-specific subnetworks and to prioritize important regulators of host response. We predicted several novel regulators, both at the mRNA and protein levels, and experimentally verified their role in the virus life cycle based on their ability to significantly impact viral replication.
Over the past 2 decades, several novel influenza virus proteins have been identified that modulate viral infections in vitro and/or in vivo. The PB2 segment, which is one of the longest influenza A virus segments, is known to encode only one viral protein, PB2. In the present study, we used reverse transcription-PCR (RT-PCR) targeting viral mRNAs transcribed from the PB2 segment to look for novel viral proteins encoded by spliced mRNAs. We identified a new viral protein, PB2-S1, encoded by a novel spliced mRNA in which the region corresponding to nucleotides 1513 to 1894 of the PB2 mRNA is deleted. PB2-S1 was detected in virus-infected cells and in cells transfected with a protein expression plasmid encoding PB2. PB2-S1 localized to mitochondria, inhibited the RIG-I-dependent interferon signaling pathway, and interfered with viral polymerase activity (dependent on its PB1-binding capability). The nucleotide sequences around the splicing donor and acceptor sites for PB2-S1 were highly conserved among pre-2009 human H1N1 viruses but not among human H1N1pdm and H3N2 viruses. PB2-S1-deficient viruses, however, showed growth kinetics in MDCK cells and virulence in mice similar to those of wild-type virus. The biological significance of PB2-S1 to the replication and pathogenicity of seasonal H1N1 influenza A viruses warrants further investigation.
IMPORTANCE Transcriptome analysis of cells infected with influenza A virus has improved our understanding of the host response to viral infection, because such analysis yields considerable information about both in vitro and in vivo viral infections. However, little attention has been paid to transcriptomes derived from the viral genome. Here we focused on the splicing of mRNA expressed from the PB2 segment and identified a spliced viral mRNA encoding a novel viral protein. This result suggests that other, as yet unidentified viral proteins encoded by spliced mRNAs could be expressed in virus-infected cells. A viral transcriptome including the viral spliceosome should be evaluated to gain new insights into influenza virus infection.
Influenza A viruses cause respiratory infections that range from asymptomatic to deadly in humans. Widespread outbreaks (pandemics) are attributable to ‘novel’ viruses that possess a viral hemagglutinin (HA) gene to which humans lack immunity. After a pandemic, these novel viruses form stable virus lineages in humans and circulate until they are replaced by other novel viruses. The factors and mechanisms that facilitate virus transmission among hosts and the establishment of novel lineages are not completely understood, but the HA and basic polymerase 2 (PB2) proteins are thought to play essential roles in these processes by enabling avian influenza viruses to infect mammals and replicate efficiently in their new host. Here, we summarize our current knowledge of the contributions of HA, PB2, and other viral components to virus transmission and the formation of new virus lineages.
Influenza virus; transmission; virus lineage; HA; PB2; NA; receptor-binding; gain-of-function
We present systemsDock, a web server for network pharmacology-based prediction and analysis, which permits docking simulation and molecular pathway map for comprehensive characterization of ligand selectivity and interpretation of ligand action on a complex molecular network. It incorporates an elaborately designed scoring function for molecular docking to assess protein–ligand binding potential. For large-scale screening and ease of investigation, systemsDock has a user-friendly GUI interface for molecule preparation, parameter specification and result inspection. Ligand binding potentials against individual proteins can be directly displayed on an uploaded molecular interaction map, allowing users to systemically investigate network-dependent effects of a drug or drug candidate. A case study is given to demonstrate how systemsDock can be used to discover a test compound's multi-target activity. systemsDock is freely accessible at http://systemsdock.unit.oist.jp/.
Ebola virus (EBOV) initially targets monocytes and macrophages, which can lead to the release of proinflammatory cytokines and chemokines. These inflammatory cytokines are thought to contribute to the development of circulatory shock seen in fatal EBOV infections. The VP40 matrix protein is a key viral structural protein that is critical for virion egress. Physical and functional interactions between VP40 and host proteins such as Tsg101 and Nedd4 facilitate efficient release of VP40-driven virus-like particles (VLPs) and infectious virus. Here, we show that host suppressor of cytokine signaling 3 (SOCS3) can also bind to EBOV VP40, leading to enhanced ubiquitinylation and egress of VP40. Indeed, titers of infectious EBOV derived from SOCS3 knockout mouse embryonic fibroblasts (MEFs) were significantly reduced compared to those from wild-type (WT) MEFs at 24 and 48 h postinfection. Importantly, this reduced virus yield could be rescued back to WT levels by exogenously expressing SOCS3. Lastly, we show that SOCS3 expression is induced by EBOV glycoprotein (GP) expression and that VLPs containing EBOV VP40 and GP induced production of proinflammatory cytokines, which induced SOCS3 for negative-feedback regulation. These data indicate that host innate immune protein SOCS3 may play an important role in budding and pathogenesis of EBOV.
IMPORTANCE The VP40 matrix protein is a key structural protein critical for Ebola virus budding. Physical and functional interactions between VP40 and host proteins such as Tsg101 and Nedd4 facilitate efficient release of VLPs and infectious virus. We reported that host TLR4 is a sensor for Ebola GP on VLPs and that the resultant TLR4 signaling pathways lead to the production of proinflammatory cytokines. Host SOCS3 regulates the innate immune response by controlling and limiting the proinflammatory response through negative-feedback inhibition of cytokine receptors. We present evidence that Ebola virus VLPs stimulate induction of SOCS3 as well as proinflammatory cytokines, and that expression of human SOCS3 enhances budding of Ebola VLPs and infectious virus via a mechanism linked to the host ubiquitinylation machinery.
We previously reported that an H5N1 virus carrying the Venus reporter gene, which was inserted into the NS gene segment from the A/Puerto Rico/8/1934(H1N1) virus (Venus-H5N1 virus), became more lethal to mice, and the reporter gene was stably maintained after mouse adaptation compared with the wild-type Venus-H5N1 (WT-Venus-H5N1) virus. However, the basis for this difference in virulence and Venus stability was unclear. Here, we investigated the molecular determinants behind this virulence and reporter stability by comparing WT-Venus-H5N1 virus with a mouse-adapted Venus-H5N1 (MA-Venus-H5N1) virus. To determine the genetic basis for these differences, we used reverse genetics to generate a series of reassortants of these two viruses. We found that reassortants with PB2 from MA-Venus-H5N1 (MA-PB2), MA-PA, or MA-NS expressed Venus more stably than did WT-Venus-H5N1 virus. We also found that a single mutation in PB2 (V25A) or in PA (R443K) increased the virulence of the WT-Venus-H5N1 virus in mice and that the presence of both of these mutations substantially enhanced the pathogenicity of the virus. Our results suggest roles for PB2 and PA in the stable maintenance of a foreign protein as an NS1 fusion protein in influenza A virus.
IMPORTANCE The ability to visualize influenza viruses has far-reaching benefits in influenza virus research. Previously, we reported that an H5N1 virus bearing the Venus reporter gene became more pathogenic to mice and that its reporter gene was more highly expressed and more stably maintained after mouse adaptation. Here, we investigated the molecular determinants behind this enhanced virulence and reporter stability. We found that mutations in PB2 (V25A) and PA (R443K) play crucial roles in the stable maintenance of a foreign protein as an NS1 fusion protein in influenza A virus and in the virulence of influenza virus in mice. Our findings further our knowledge of the pathogenicity of influenza virus in mammals and will help advance influenza virus-related live-imaging studies in vitro and in vivo.
Influenza A virus PA-X comprises an N-terminal PA endonuclease domain and a C-terminal PA-X-specific domain. PA-X reduces host and viral mRNA accumulation via its endonuclease function. Here, we found that the N-terminal 15 amino acids, particularly six basic amino acids, in the C-terminal PA-X-specific region are important for PA-X shutoff activity. These six basic amino acids enabled a PA deletion mutant to suppress protein expression at a level comparable to that of wild-type PA-X.
Influenza viruses that express reporter proteins are useful tools, but are often attenuated. Recently, we found that an influenza virus encoding the Venus fluorescent protein acquired two mutations in its PB2 and HA proteins upon mouse adaptation. Here, we demonstrate that the enhanced viral replication and virulence in mice of this Venus-expressing influenza virus are primarily conferred by the PB2-E712D mutation, with only a minor contribution by the HA-T380A mutation.
Host factors required for viral replication are ideal drug targets because they are less likely than viral proteins to mutate under drug-mediated selective pressure. Although genome-wide screens have identified host proteins involved in influenza virus replication, limited mechanistic understanding of how these factors affect influenza has hindered potential drug development. We conducted a systematic analysis to identify and validate host factors that associate with influenza virus proteins and affect viral replication. After identifying over one thousand host factors that co-immunoprecipitate with specific viral proteins, we generated a network of virus-host protein interactions based on the stage of the viral lifecycle affected upon host factor down-regulation. Using compounds that inhibit these host factors, we validated several proteins, notably Golgi-specific brefeldin A resistant guanine nucleotide exchange factor (GBF1) and JAK1, as potential antiviral drug targets. Thus, virus-host interactome screens are powerful strategies to identify targetable host factors and guide antiviral drug development.
Avian influenza viruses rarely infect humans, but the recently emerged avian H7N9 influenza viruses have caused sporadic infections in humans in China, resulting in 440 confirmed cases with 122 fatalities as of May 16, 2014. In addition, epidemiologic surveys suggest that there have been asymptomatic or mild human infections with H7N9 viruses. These viruses replicate efficiently in mammals, show limited transmissibility in ferrets and guinea pigs, and possess mammalian-adapting amino acid changes that likely contribute to their ability to infect mammals. Here, we summarize the characteristic features of the novel H7N9 viruses and assess their pandemic potential.
Avian influenza H7N9 viruses; transmission; pandemic potential
Between September 2013 and July 2014, 2,482 influenza 2009 pandemic A(H1N1) [A(H1N1)pdm09] viruses were screened in Japan for the H275Y substitution in their neuraminidase (NA) protein, which confers cross-resistance to oseltamivir and peramivir. We found that a large cluster of the H275Y mutant virus was present prior to the main influenza season in Sapporo/Hokkaido, with the detection rate for this mutant virus reaching 29% in this area. Phylogenetic analysis suggested the clonal expansion of a single mutant virus in Sapporo/Hokkaido. To understand the reason for this large cluster, we examined the in vitro and in vivo properties of the mutant virus. We found that it grew well in cell culture, with growth comparable to that of the wild-type virus. The cluster virus also replicated well in the upper respiratory tract of ferrets and was transmitted efficiently between ferrets by way of respiratory droplets. Almost all recently circulating A(H1N1)pdm09 viruses, including the cluster virus, possessed two substitutions in NA, V241I and N369K, which are known to increase replication and transmission fitness. A structural analysis of NA predicted that a third substitution (N386K) in the NA of the cluster virus destabilized the mutant NA structure in the presence of the V241I and N369K substitutions. Our results suggest that the cluster virus retained viral fitness to spread among humans and, accordingly, caused the large cluster in Sapporo/Hokkaido. However, the mutant NA structure was less stable than that of the wild-type virus. Therefore, once the wild-type virus began to circulate in the community, the mutant virus could not compete and faded out.
Highly pathogenic H5N1 influenza A viruses continue to circulate among avian species and cause sporadic cases of human infection. Therefore, the threat of a pandemic persists. However, the human cases of H5N1 infection have been limited mainly to individuals in close contact with infected poultry. These findings suggest that the H5N1 viruses need to acquire adaptive mutations to gain a replicative advantage in mammalian cells to break through the species barrier. Many amino acid mutations of the polymerase complex have been reported to enhance H5N1 virus growth in mammalian cells; however, the mechanism for H5N1 virus of adaptation to humans remains unclear. Here, we propose that the PA of an H5N1 influenza virus isolated from a human in Vietnam (A/Vietnam/UT36285/2010 ) increased the ability of an avian H5N1 virus (A/chicken/Vietnam/TY31/2005 [Ck/TY31]) to grow in human lung epithelial A549 cells. The five PA amino acid substitutions V44I, V127A, C241Y, A343T, and I573V, which are rare in H5N1 viruses from human and avian sources, enhanced the growth capability of this virus in A549 cells. Moreover, these mutations increased the pathogenicity of the virus in mice, suggesting that they contribute to adaptation to mammalian hosts. Intriguingly, PA-241Y, which 36285 encodes, is conserved in more than 90% of human seasonal H1N1 viruses, suggesting that PA-241Y contributes to virus adaptation to human lung cells and mammalian hosts.
IMPORTANCE Many amino acid substitutions in highly pathogenic H5N1 avian influenza viruses have been shown to contribute to adaptation to mammalian hosts. However, no naturally isolated H5N1 virus has caused extensive human-to-human transmission, suggesting that additional, as-yet unidentified amino acid mutations are needed for adaptation to humans. Here, we report that five amino acid substitutions in PA (V44I, V127A, C241Y, A343T, and I573V) contribute to the replicative efficiency of H5N1 viruses in human lung cells and to high virulence in mice. These results are helpful for assessing the pandemic risk of isolates and further our understanding of the mechanism of H5N1 virus adaptation to mammalian hosts.
Highly pathogenic H5N1 avian influenza viruses have caused outbreaks among poultry worldwide, resulting in sporadic infections in humans with approximately 60% mortality. However, efficient transmission of H5N1 viruses among humans has yet to occur, suggesting that further adaptation of H5N1 viruses to humans is required for their efficient transmission among humans. The viral determinants for efficient replication in humans are currently poorly understood. Here, we report that the polymerase PB2 protein of an H5N1 influenza virus isolated from a human in Vietnam (A/Vietnam/UT36285/2010, virus 36285) increased the growth ability of an avian H5N1 virus (A/wild bird/Anhui/82/2005, virus Wb/AH82) in human lung epithelial A549 cells (however, the reassortant virus did not replicate more efficiently than human 36285 virus). Furthermore, we demonstrate that the amino acid residues at positions 249, 309, and 339 of the PB2 protein from this human isolate were responsible for its efficient replication in A549 cells. PB2 residues 249G and 339M, which are found in the human H5N1 virus, are rare in H5N1 viruses from both human and avian sources. Interestingly, PB2-249G is found in over 30% of human seasonal H3N2 viruses, which suggests that H5N1 viruses may replicate well in human cells when they acquire this mutation. Our data are of value to H5N1 virus surveillance.
IMPORTANCE Highly pathogenic H5N1 avian influenza viruses must acquire mutations to overcome the species barrier between avian species and humans. When H5N1 viruses replicate in human respiratory cells, they can acquire amino acid mutations that allow them to adapt to humans through continuous selective pressure. Several amino acid mutations have been shown to be advantageous for virus adaptation to mammalian hosts. Here, we found that amino acid changes at positions 249, 309, and 339 of PB2 contribute to efficient replication of avian H5N1 viruses in human lung cells. These findings are beneficial for evaluating the pandemic risk of circulating avian viruses and for further functional analysis of PB2.
We compared the sensitivity of influenza rapid diagnostic tests (IRDTs) currently available in Japan for various influenza virus strains, including human H7N9 and H5N1 isolates. We found that all of the IRDTs examined detected these viruses, but their detection sensitivities differed.
Influenza; rapid diagnostic tests; detection sensitivity; avian H7N9 and H5N1 viruses