Impact of potential permissive neuraminidase mutations on viral fitness of the H275Y oseltamivir-resistant influenza A(H1N1)pdm09 virus in vitro, in mice and in ferrets

Integrated virtual screening and compound generation targeting H275Y mutation in the neuraminidase gene of oseltamivir-resistant influenza strains

Neuraminidase (NA) is an essential enzyme located at the outer layer of the influenza virus and plays a key role in the release of virions from infected cells. The rising incidence of global epidemics has made the urgent need for effective antiviral medications an urgent public health priority. Furthermore, the emergence of resistance caused by specific mutations in the influenza viral genome exacerbates the challenges of antiviral therapy. In view of this, this study aims to identify and analyse possible inhibitors of NA from different subtypes of influenza viruses. Initially, a thorough search was conducted in the Protein Data Bank (PDB) to gather structures of NA proteins that were attached with oseltamivir, a widely recognized inhibitor of NA. Here, 36 PDB entries were found with NA-oseltamivir complexes which were studied to evaluate the diversity and mutations present in various subtypes. Finally, N1(H1N1) protein was selected that demonstrated low IC50 value of oseltamivir with mutation H275Y. In addition, the study utilized BiMODAL generative model to generate 1000 novel molecules with comparable structures to oseltamivir. A QSAR model, based on machine learning (ML), was built utilizing the ChEMBL database to improve the selection process of candidate inhibitors. These inhibitors were subsequently analysed by molecular docking and further the best hits compounds (compound_375, compound_106 and compound_597) were appended to make a bigger molecule (compound_106-375, compound_106-597, and compound_375-597) to fit into the binding pocket of protein. Further, triplicate molecular dynamics simulations lasting 100 ns to assess their effectiveness and binding stability showed that compound_106-375 had the most stable binding with the protein. Key residues, including Asn146, Ala138, and Tyr155, form critical interactions with the ligand, contributing to its stability. The investigation was enhanced by employing principal component analysis (PCA), free energy landscape (FEL), and binding free energy calculations. The total binding free energy (GTOTAL) of - 169.62 kcal/mol suggests that the contact between compound_106-375 and the mutant N1 (H1N1) protein is thermodynamically favourable. This approach allowed for a thorough comprehension of the binding interactions and possible effectiveness of the discovered inhibitors. Overall, these findings demonstrate that compound_106-375 exhibits favourable binding characteristics and stability. Further experimental validation is required to confirm its efficacy against the H275Y mutant neuraminidase protein and its potential to overcome influenza drug resistance. However, compound_106-375 is suggested as a promising candidate for further development as a therapeutic agent against the mutant N1 (H1N1) protein. This finding will assist in drug development and to overcome the challenges associated with drug resistance in influenza strains.

Increase of Synergistic Secondary Antiviral Mutations in the Evolution of A(H1N1)pdm09 Influenza Virus Neuraminidases

The unexpected emergence of oseltamivir-resistant A(H1N1) viruses in 2008 was facilitated in part by the establishment of permissive secondary neuraminidase (NA) substitutions that compensated for the fitness loss due to the NA-H275Y resistance substitution. These viruses were replaced in 2009 by oseltamivir-susceptible A(H1N1)pdm09 influenza viruses. Genetic analysis and screening of A(H1N1)pdm09 viruses circulating in Germany between 2009 and 2024 were conducted to identify any potentially synergistic or resistance-associated NA substitutions. Selected viruses were then subjected to further characterization in vitro. In the NA gene of circulating A(H1N1)pdm09 viruses, two secondary substitutions, NA-V241I and NA-N369K, were identified. These substitutions demonstrated a stable lineage in phylogenetic analysis since the 2010-2011 influenza season. The data indicate a slight increase in viral NA bearing two additional potentially synergistic substitutions, NA-I223V and NA-S247N, in the 2023-2024 season, which both result in a slight reduction in susceptibility to NA inhibitors. The accumulation of secondary synergistic substitutions in the NA of A(H1N1)pdm09 viruses increases the probability of the emergence of antiviral-resistant viruses. Therefore, it is crucial to closely monitor the evolution of circulating influenza viruses and to develop additional antiviral drugs against different target proteins.

Impact of the H274Y Substitution on N1, N4, N5, and N8 Neuraminidase Enzymatic Properties and Expression in Reverse Genetic Influenza A Viruses

The H274Y substitution (N2 numbering) in neuraminidase (NA) N1 confers oseltamivir resistance to A(H1N1) influenza viruses. This resistance has been associated with reduced N1 expression using transfected cells, but the effect of this substitution on the enzymatic properties and on the expression of other group-1-NA subtypes is unknown. The aim of the present study was to evaluate the antiviral resistance, enzymatic properties, and expression of wild-type (WT) and H274Y-substituted NA for each group-1-NA. To this end, viruses with WT or H274Y-substituted NA (N1pdm09 or avian N4, N5 or N8) were generated by reverse genetics, and for each reverse-genetic virus, antiviral susceptibility, NA affinity (Km), and maximum velocity (Vm) were measured. The enzymatic properties were coupled with NA quantification on concentrated reverse genetic viruses using mass spectrometry. The H274Y-NA substitution resulted in highly reduced inhibition by oseltamivir and normal inhibition by zanamivir and laninamivir. This resistance was associated with a reduced affinity for MUNANA substrate and a conserved Vm in all viruses. NA quantification was not significantly different between viruses carrying WT or H274Y-N1, N4 or N8, but was lower for viruses carrying H274Y-N5 compared to those carrying a WT-N5. In conclusion, the H274Y-NA substitution of different group-1-NAs systematically reduced their affinity for MUNANA substrate without a significant impact on NA Vm. The impact of the H274Y-NA substitution on viral NA expression was different according to the studied NA.

Alternative antiviral approaches to combat influenza A virus

Influenza A (IAV) is a major human respiratory pathogen that contributes to a significant threat to health security, worldwide. Despite vaccinations and previous immunisations through infections, humans can still be infected with influenza several times throughout their lives. This phenomenon is attributed to the antigenic changes of hemagglutinin (HA) and neuraminidase (NA) proteins in IAV via genetic mutation and reassortment, conferring antigenic drift and antigenic shift, respectively. Numerous findings indicate that slow antigenic drift and reassortment-derived antigenic shift exhibited by IAV are key processes that allow IAVs to overcome the previously acquired host immunity, which eventually leads to the annual re-emergence of seasonal influenza and even pandemic influenza, in rare occasions. As a result, current therapeutic options hit a brick wall quickly. As IAV remains a constant threat for new outbreaks worldwide, the underlying processes of genetic changes and alternative antiviral approaches for IAV should be further explored to improve disease management. In the light of the above, this review discusses the characteristics and mechanisms of mutations and reassortments that contribute to IAV's evolution. We also discuss several alternative RNA-targeting antiviral approaches, namely the CRISPR/Cas13 systems, RNA interference (RNAi), and antisense oligonucleotides (ASO) as potential antiviral approaches against IAV.

Predicting Permissive Mutations That Improve the Fitness of A(H1N1)pdm09 Viruses Bearing the H275Y Neuraminidase Substitution

Oseltamivir-resistant influenza viruses arise due to amino acid mutations in key residues of the viral neuraminidase (NA). These changes often come at a fitness cost; however, it is known that permissive mutations in the viral NA can overcome this cost. This result was observed in former seasonal A(H1N1) viruses in 2007 which expressed the H275Y substitution (N1 numbering) with no apparent fitness cost and lead to widespread oseltamivir resistance. Therefore, this study aims to predict permissive mutations that may similarly enable fit H275Y variants to arise in currently circulating A(H1N1)pdm09 viruses. The first approach in this study utilized in silico analyses to predict potentially permissive mutations. The second approach involved the generation of a virus library which encompassed all possible NA mutations while keeping H275Y fixed. Fit variants were then selected by serially passaging the virus library either through ferrets by transmission or passaging once in vitro. The fitness impact of selected substitutions was further evaluated experimentally. The computational approach predicted three candidate permissive NA mutations which, in combination with each other, restored the replicative fitness of an H275Y variant. The second approach identified a stringent bottleneck during transmission between ferrets; however, three further substitutions were identified which may improve transmissibility. A comparison of fit H275Y variants in vitro and in experimentally infected animals showed a statistically significant correlation in the variants that were positively selected. Overall, this study provides valuable tools and insights into potential permissive mutations that may facilitate the emergence of a fit H275Y A(H1N1)pdm09 variant.

IMPORTANCE Oseltamivir (Tamiflu) is the most widely used antiviral for the treatment of influenza infections. Therefore, resistance to oseltamivir is a public health concern. This study is important as it explores the different evolutionary pathways available to current circulating influenza viruses that may lead to widespread oseltamivir resistance. Specifically, this study develops valuable experimental and computational tools to evaluate the fitness landscape of circulating A(H1N1)pmd09 influenza viruses bearing the H275Y mutation. The H275Y substitution is most commonly reported to confer oseltamivir resistance but also leads to loss of virus replication and transmission fitness, which limits its spread. However, it is known from previous influenza seasons that influenza viruses can evolve to overcome this loss of fitness. Therefore, this study aims to prospectively predict how contemporary A(H1N1)pmd09 influenza viruses may evolve to overcome the fitness cost of bearing the H275Y NA substitution, which could result in widespread oseltamivir resistance.

Frequency and distribution of H1N1 influenza A viruses with oseltamivir-resistant mutations worldwide before and after the 2009 pandemic

H1N1 influenza has brought serious threats to people's health and a high socio-economic burden to society. Oseltamivir, a kind of neuraminidase (NA) inhibitor, is the second-generation specific drug that is broadly used currently. However, H1N1 influenza viruses have exhibited oseltamivir resistance in the past decades, which might be a hidden danger. To understand the frequency and distribution laws of oseltamivir-resistant viruses, we conducted a thorough and deep analysis of the available NA protein sequences of H1N1 influenza viruses worldwide from 1918 to 2020. The differences and similarities before and after 2009 were also considered since the dominant viruses changed in this period. Results showed that 3.76% of H1N1 viruses harbored oseltamivir resistance currently. Among various significative mutations, H274Y had the highest frequency of 3.30%, while the frequencies of the other mutations were far below this whether before or after 2009. The oseltamivir resistance was mainly found in three hosts, human, swine, and avian. Different mutation sites could exhibit different distributions in each host. Our results showed that the resistance level reached a peak during the 2007-2008 influenza season and then quickly decreased in 2009. The resistance also displayed a global distribution. The densely populated countries usually had a high resistance level. However, frequent significative mutations were also found in some small countries. Our findings indicated the necessity of monitoring oseltamivir resistance around the world. The study could provide a unique perspective towards the cognition of viruses and facilitate the future study of both pandemic and drug development. This article is protected by copyright. All rights reserved.

Identification of a permissive secondary mutation that restores the enzymatic activity of oseltamivir resistance mutation H275Y

Many oseltamivir resistance mutations exhibit fitness defects in the absence of drug pressure that hinders their propagation in hosts. Secondary permissive mutations can rescue fitness defects and facilitate the segregation of resistance mutations in viral populations. Previous studies have identified a panel of permissive or compensatory mutations in neuraminidase (NA) that restore the growth defect of the predominant oseltamivir resistance mutation (H275Y) in H1N1 influenza A. In prior work, we identified a hyperactive mutation (Y276F) that increased NA activity by approximately 70%. While Y276F had not been previously identified as a permissive mutation, we hypothesized that Y276F may counteract the defects caused by H275Y by buffering its reduced NA expression and enzyme activity. In this study we measured the relative fitness, NA activity, and surface expression, as well as sensitivity to oseltamivir, for several oseltamivir resistance mutations including H275Y in the wildtype or Y276F genetic background. Our results demonstrate that Y276F selectively rescues the fitness defect of H275Y by restoring its NA surface expression and enzymatic activity, elucidating the local compensatory structural impacts of Y276F on the adjacent H275Y. Importance The potential for influenza A virus (IAV) to cause pandemics makes understanding evolutionary mechanisms that impact drug resistance critical for developing surveillance and treatment strategies. Oseltamivir is the most widely used therapeutic strategy to treat IAV infections, but mutations in IAV can lead to drug resistance. The main oseltamivir resistance mutation, H275Y, occurs in the neuraminidase (NA) protein of IAV and reduces drug binding as well as NA function. Here, we identify a new helper mutation, Y276F that can rescue the functional defects of H275Y and contribute to the evolution of drug resistance in IAV.

Dual oxidase 1 promotes antiviral innate immunity

Dual oxidase 1 (DUOX1) is an NADPH oxidase that is highly expre-ssed in respiratory epithelial cells and produces H2O2 in the airway lumen. While a line of prior in vitro observations suggested that DUOX1 works in partnership with an airway peroxidase, lactoperoxidase (LPO), to produce antimicrobial hypothiocyanite (OSCN-) in the airways, the in vivo role of DUOX1 in mammalian organisms has remained unproven to date. Here, we show that Duox1 promotes antiviral innate immunity in vivo. Upon influenza airway challenge, Duox1-/- mice have enhanced mortality, morbidity, and impaired lung viral clearance. Duox1 increases the airway levels of several cytokines (IL-1β, IL-2, CCL1, CCL3, CCL11, CCL19, CCL20, CCL27, CXCL5, and CXCL11), contributes to innate immune cell recruitment, and affects epithelial apoptosis in the airways. In primary human tracheobronchial epithelial cells, OSCN- is generated by LPO using DUOX1-derived H2O2 and inactivates several influenza strains in vitro. We also show that OSCN- diminishes influenza replication and viral RNA synthesis in infected host cells that is inhibited by the H2O2 scavenger catalase. Binding of the influenza virus to host cells and viral entry are both reduced by OSCN- in an H2O2-dependent manner in vitro. OSCN- does not affect the neuraminidase activity or morphology of the influenza virus. Overall, this antiviral function of Duox1 identifies an in vivo role of this gene, defines the steps in the infection cycle targeted by OSCN-, and proposes that boosting this mechanism in vivo can have therapeutic potential in treating viral infections.

Molecular evolution and characterization of hemagglutinin and neuraminidase of influenza A(H1N1)pdm09 viruses isolated in Beijing, China, during the 2017-2018 and 2018-2019 influenza seasons

We investigated and analysed the molecular evolution of hemagglutinin (HA) and neuraminidase (NA) of influenza A(H1N1)pdm09 virus during the 2017-2018 and 2018-2019 influenza seasons in Beijing, China. We collected and extracted RNA from influenza A(H1N1)pdm09 strains from Peking University People's Hospital and analyzed their HA and NA genes by RT-PCR and sequencing. Phylogenetic analysis of HA and NA sequences was used to compare the amino acid sequences of 51 strains with those of reference strains. All strains belonged to subclade 6B.1, with S162N and I216T substitutions (H1 numbering). Our strains differed from strain A/Michigan/45/2015, with the substitutions S91R, S181T and I312V in the HA antigenic epitope. An E189G mutation was detected in the 190 helix of the receptor binding region of HA. A new potential glycosylation site, 179 (NQT), which was not detected before the 2015 influenza season, was identified. Two strains were mutated at I223, the NA inhibitor resistance site. During 2012-2019, amino acids of HA and NA mutated over time. Co-occurrence mutations N146D, S200P, S202I and A273T in HA appeared along with Q51K, F74S and D416N in NA in six strains during two influenza seasons. Our work reveals the molecular changes and phylogenetic characteristics of influenza A(H1N1)pdm09 virus and suggests that a vaccine probably provides suboptimal protection. The biological characteristics of the new glycosylation and drug-resistance sites detected in this work need to be studied further. The co-occurrence of mutations in HA and NA might affect the characteristics of the virus and need to be given more attention.

In Vitro Characterization of Multidrug-Resistant Influenza A(H1N1)pdm09 Viruses Carrying a Dual Neuraminidase Mutation Isolated from Immunocompromised Patients

Influenza A(H1N1)pdm09 viruses carrying a dual neuraminidase (NA) substitution were isolated from immunocompromised patients after administration of one or more NA inhibitors. These mutant viruses possessed an H275Y/I223R, H275Y/I223K, or H275Y/G147R substitution in their NA and showed enhanced cross-resistance to oseltamivir and peramivir and reduced susceptibility to zanamivir compared to single H275Y mutant viruses. Baloxavir could be a treatment option against the multidrug-resistant viruses because these dual H275Y mutant viruses showed susceptibility to this drug. The G147R substitution appears to stabilize the NA structure, with the fitness of the H275Y/G147R mutant virus being similar or somewhat better than that of the wild-type virus. Since the multidrug-resistant viruses may be able to transmit between humans, surveillance of these viruses must continue to improve clinical management and to protect public health.

Differential Viral-Host Immune Interactions Associated with Oseltamivir-Resistant H275Y and Wild-Type H1N1 A(pdm09) Influenza Virus Pathogenicity

Oseltamivir is a common therapy against influenza A virus (IAV) infections. The acquisition of oseltamivir resistance (OR) mutations, such as H275Y, hampers viral fitness. However, OR H1N1 viruses have demonstrated the ability to spread throughout different populations. The objective of this work was to compare the fitness of two strains of OR (R6 and R7) containing the H275Y mutation, and a wild-type (F) pandemic influenza A (H1N1) 2009 (pdm09) virus both in vitro and in vivo in mice and to select one OR strain for a comparison with F in ferrets. R6 showed faster replication and pathogenicity than R7 in vitro and in mice. Subsequently, R6 was selected for the fitness comparison with the F strain in ferrets. Ferrets infected with the F virus showed more severe clinical signs, histopathological lung lesions, and viral quantification when compared to OR R6-infected animals. More importantly, differential viral kinetics correlated with differential pro-inflammatory host immune responses in the lungs of infected ferrets, where OR-infected animals developed a protective higher expression of type I IFN and Retinoid acid Inducible Gene I (RIG-I) genes early after infection, resulting in the development of milder disease. These results suggest the presence of early specific viral-host immune interactions relevant in the development of influenza-associated lung pathology.

Comparative review of respiratory diseases caused by coronaviruses and influenza A viruses during epidemic season

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to sweep the world, causing infection of millions and death of hundreds of thousands. The respiratory disease that it caused, COVID-19 (stands for coronavirus disease in 2019), has similar clinical symptoms with other two CoV diseases, severe acute respiratory syndrome and Middle East respiratory syndrome (SARS and MERS), of which causative viruses are SARS-CoV and MERS-CoV, respectively. These three CoVs resulting diseases also share many clinical symptoms with other respiratory diseases caused by influenza A viruses (IAVs). Since both CoVs and IAVs are general pathogens responsible for seasonal cold, in the next few months, during the changing of seasons, clinicians and public heath may have to distinguish COVID-19 pneumonia from other kinds of viral pneumonia. This is a discussion and comparison of the virus structures, transmission characteristics, clinical symptoms, diagnosis, pathological changes, treatment and prevention of the two kinds of viruses, CoVs and IAVs. It hopes to provide information for practitioners in the medical field during the epidemic season.

Characterization of substitutions in the neuraminidase of A(H7N9) influenza viruses selected following serial passage in the presence of different neuraminidase inhibitors

Avian A(H7N9) infections in humans have been reported in China since 2013 and are of public health concern due to their severity and pandemic potential. Oseltamivir and peramivir are neuraminidase inhibitors (NAIs) routinely used for the treatment of A(H7N9) infections, but variants with reduced sensitivity to these drugs can emerge in patients during treatment. Zanamivir and laninamivir are NAIs that are used less frequently. Herein, we performed in vitro serial passaging experiments with recombinant viruses, containing the neuraminidase (NA) from influenza A/Anhui/1/13 (H7N9) virus, in the presence of each NAI, to determine whether variants with reduced sensitivity would emerge. NA substitutions were characterized for their effect on the NA enzymatic activity and surface expression of the A/Anhui/1/13 (Anhui/1) NA, as well as NAs originating from contemporary A(H7N9) viruses of the Yangtze River Delta and Pearl River Delta lineages. In vitro passage in the presence of oseltamivir, peramivir and laninamivir selected for substitutions associated with reduced sensitivity (E119D, R292K and R152K), whereas passage in the presence of zanamivir did not select for any viruses with reduced sensitivity. All the NA substitutions significantly reduced activity, but not the expression of the Anhui/1 NA. In contemporary N9 NAs, all substitutions tested significantly reduced NA enzyme function in the Yangtze River lineage background, but not in the Pearl River Delta lineage background. Overall, these findings suggest that zanamivir may be less likely than the other NAIs to select for resistance in A(H7N9) viruses and that the impact of substitutions that reduce NAI susceptibility or enzyme function may be less in A(H7N9) viruses from the Pearl River lineage.

Detection of neuraminidase inhibitor-resistant influenza A (H1N1)pdm09 viruses obtained from influenza surveillance in Indonesia

Background:

Influenza antiviral resistance has been shown to occur in many countries and is commonly found in influenza A(H1N1)pdm09 and A(H3N2). In this study, we monitored and investigated the neuraminidase inhibitor resistance of influenza A(H1N1)pdm09 viruses through the influenza surveillance system in Indonesia.

Methods:

A total of 4752 clinical specimens were collected from patients with influenza-like illness and severe acute respiratory infection during the year 2016. An allelic discrimination assay was conducted by a single base substitution or a single-nucleotide polymorphism that is specific to the H275 wild-type and Y275 mutant. Sequencing was performed to confirm the H275Y mutations, and we analysed the phylogenetic relationship.

Results:

The first occurrence of oseltamivir-resistant influenza A(H1N1)pdm09 was observed in the samples from the influenza-like illness surveillance. Two H275Y oseltamivir-resistant viruses (0.74%) out of 272 influenza A(H1N1)pdm09 positives were found. Both of them were collected from untreated patients.

Conclusion:

The number of oseltamivir-resistant influenza A(H1N1)pdm09 viruses in Indonesia is very low. However, it is necessary to continue with active surveillance for oseltamivir resistance in severe and mild cases.

Susceptibility of Influenza Viruses to the Novel Cap-Dependent Endonuclease Inhibitor Baloxavir Marboxil

The novel cap-dependent endonuclease inhibitor baloxavir marboxil was approved for the treatment of influenza A and B virus infections in February 2018 in Japan. Because of the need to monitor influenza viruses for reduced susceptibility to this drug, we used two cell-based screening systems - a conventional plaque reduction assay and a focus reduction assay - to evaluate the susceptibility of influenza viruses to baloxavir. First, we generated a reference virus possessing an I38T substitution in the polymerase acidic subunit (PA), which is known to be associated with reduced susceptibility to baloxavir, and demonstrated the validity of our systems using this reference virus. We then determined the susceptibility of a panel of neuraminidase (NA) inhibitor-resistant viruses and their sensitive counterparts to baloxavir. No significant differences in baloxavir susceptibilities were found between the NA inhibitor-resistant and -sensitive viruses. We also examined seasonal influenza viruses isolated during the 2017-2018 influenza season in Japan and found that no currently circulating A(H1N1)pdm09, A(H3N2), or B viruses had significantly reduced susceptibility to baloxavir and none of the viruses possessed an amino acid substitution at PA residue 38. Use of a combination of methods to analyze antiviral susceptibility and detect amino acid substitutions is valuable for monitoring the emergence of baloxavir-resistant viruses.

Comparison of early and recent influenza A(H1N1)pdm09 isolates harboring or not the H275Y neuraminidase mutation, in vitro and in animal models

After 6 years of circulation in humans, a novel antigenic variant of influenza A(H1N1)pdm09 (i.e., A/Michigan/45/2015) emerged in 2015-16 and has predominated thereafter worldwide. Herein, we compared in vitro and in vivo properties of 2016 wild-type (WT) A/Michigan/45/15-like isolate and its H275Y neuraminidase (NA) variant to the original A/California/07/09-like counterparts. The H275Y mutation induced comparable levels of resistance to oseltamivir and peramivir without altering zanamivir susceptibility in both 2009 and 2016 isolates. In vitro, the two WT isolates had comparable replicative properties. The 2016-H275Y isolate had lower titers at 36 h post-inoculation (PI) (P < 0.05) while the 2009-H275Y titers were lower at both 24 h (P < 0.01) and 36 h PI (P < 0.001) vs the respective WTs. In mice, the 2016-WT isolate caused less weight losses (P < 0.001) and lower lung viral titers (LVTs) (P < 0.01) vs the 2009-WT. The LVTs of 2016-WT and 2016-H275Y groups were comparable whereas the 2009-H275Y LVTs were lower vs the respective WT (P < 0.01). Ferrets infected with the 2016-WT isolate and their contacts had higher nasal viral titers (NVTs) at early time points vs the 2009-WT group (P < 0.01). Also, NVTs of 2016-H275Y animals were lower vs the 2016-WT group at early time points in both infected (P < 0.01) and contact animals (P < 0.001). In conclusion, while the H275Y mutation similarly impacts the A/California/07/2009- and A/Michigan/45/2015-like A(H1N1)pdm09 NAs, the fitness of these isolates differs according to animal models with the 2016 virus being less virulent in mice but slightly more virulent in ferrets, potentially reflecting a period of cumulative changes in surface and internal genes.

Rapid and simple detection of Tamiflu-resistant influenza virus: Development of oseltamivir derivative-based lateral flow biosensor for point-of-care (POC) diagnostics

We have developed a novel oseltamivir derivative (oseltamivir hexylthiol; OHT) that exhibits a higher binding affinity for Tamiflu-resistant virus (Tamiflu resistance) than for the wild-type virus (Tamiflu-susceptible virus; WT) as an antibody. First, OHT-modified gold nanoparticles (OHT-GNPs) are used in a simple colorimetric assay as nanoprobes for the Tamiflu-resistant virus. In the presence of Tamiflu-resistant virus, they show a colorimetric change from deep red to purple because of the OHT-GNP aggregation driven by strong interactions between OHT and neuraminidase (NA) on the surface of the Tamiflu-resistance. Moreover, the color gradually turns purple as the concentration of the Tamiflu-resistant virus increases, allowing the determination of the presence of the virus with the naked eye. Furthermore, an OHT-based lateral flow assay (LFA) has been developed as a rapid and easy detection device for Tamiflu resistance. It shows detection specificity for various virus concentrations of Tamiflu-resistant virus even for the mixture of WT and Tamiflu-resistant viruses, where the limit of detection (LOD) is 5 × 10² ~ 10³ PFU per test (=1 × 10⁴ PFU/mL). It has been confirmed that this platform can provide accurate information on whether a virus exhibits Tamiflu resistance, thus supporting the selection of appropriate treatments using point-of-care (POC) diagnostics.

Characterization of influenza virus variants induced by treatment with the endonuclease inhibitor baloxavir marboxil

Baloxavir acid (BXA), derived from the prodrug baloxavir marboxil (BXM), potently and selectively inhibits the cap-dependent endonuclease within the polymerase PA subunit of influenza A and B viruses. In clinical trials, single doses of BXM profoundly decrease viral titers as well as alleviating influenza symptoms. Here, we characterize the impact on BXA susceptibility and replicative capacity of variant viruses detected in the post-treatment monitoring of the clinical studies. We find that the PA I38T substitution is a major pathway for reduced susceptibility to BXA, with 30- to 50-fold and 7-fold EC₅₀ changes in A and B viruses, respectively. The viruses harboring the I38T substitution show severely impaired replicative fitness in cells, and correspondingly reduced endonuclease activity in vitro. Co-crystal structures of wild-type and I38T influenza A and B endonucleases bound to BXA show that the mutation reduces van der Waals contacts with the inhibitor. A reduced affinity to the I38T mutant is supported by the lower stability of the BXA-bound endonuclease. These mechanistic insights provide markers for future surveillance of treated populations.

Modelling the emergence of influenza drug resistance: The roles of surface proteins, the immune response and antiviral mechanisms

The emergence of influenza drug resistance has become of particular interest as current planning for an influenza pandemic involves using massive amounts of antiviral drugs. We use semi-stochastic simulations to examine the emergence of drug resistant mutants during the course of a single infection within a patient in the presence and absence of antiviral therapy. We specifically examine three factors and their effect on the emergence of drug-resistant mutants: antiviral mechanism, the immune response, and surface proteins. We find that adamantanes, because they act at the start of the replication cycle to prevent infection, are less likely to produce drug-resistant mutants than NAIs, which act at the end of the replication cycle. A mismatch between surface proteins and internal RNA results in drug-resistant mutants being less likely to emerge, and emerging later in the infection because the mismatch gives antivirals a second chance to prevent propagation of the mutation. The immune response subdues slow growing infections, further reducing the probability that a drug resistant mutant will emerge and yield a drug-resistant infection. These findings improve our understanding of the factors that contribute to the emergence of drug resistance during the course of a single influenza infection.

Influenza A(H1N1)pdm09 virus exhibiting enhanced cross-resistance to oseltamivir and peramivir due to a dual H275Y/G147R substitution, Japan, March 2016

An influenza A(H1N1)pdm09 virus carrying a G147R substitution in combination with an H275Y substitution in the neuraminidase protein, which confers cross-resistance to oseltamivir and peramivir, was detected from an immunocompromised inpatient in Japan, March 2016. This dual H275Y/G147R mutant virus exhibited enhanced cross-resistance to both drugs compared with the single H275Y mutant virus and reduced susceptibility to zanamivir, although it showed normal inhibition by laninamivir.

The I427T neuraminidase (NA) substitution, located outside the NA active site of an influenza A(H1N1)pdm09 variant with reduced susceptibility to NA inhibitors, alters NA properties and impairs viral fitness

Emergence of pan neuraminidase inhibitor (NAI)-resistant variants constitutes a serious clinical concern. An influenza A(H1N1)pdm09 variant containing the I427T/Q313R neuraminidase (NA) substitutions was previously identified in a surveillance study. Although these changes are not part of the NA active site, the variant showed reduced susceptibility to many NAIs. In this study, we investigated the mechanism of resistance for the I427T/Q313R substitution and its impact on the NA enzyme and viral fitness. Recombinant wild-type (WT), I427T/Q313R and I427T A(H1N1)pdm09 viruses were generated by reverse genetics and tested for their drug susceptibilities, enzymatic properties and replication kinetics in vitro as well as their virulence in mice. Molecular dynamics (MD) simulations were performed for NA structural analysis. The I427T substitution, which was responsible for the resistance phenotype observed in the double (I427T/Q313R) mutant, induced 17-, 56-, 7-, and 14-fold increases in IC₅₀ values against oseltamivir, zanamivir, peramivir and laninamivir, respectively. The I427T substitution alone or combined to Q313R significantly reduced NA affinity. The I427T/Q313R and to a lesser extent I427T recombinant viruses displayed reduced viral titers vs WT in vitro. In experimentally-infected mice, the mortality rates were 62.5%, 0% and 14.3% for the WT, I417T/Q313R and I427T viruses, respectively. There were about 2.5- and 2-Log reductions in mean lung viral titers on day 5 post-infection for the I427T/Q313R and I427T mutants, respectively, compared to WT. Results from simulations revealed that the I427T change indirectly altered the stability of the catalytic R368 residue of the NA enzyme causing its reduced binding to the substrate/inhibitor. This study demonstrates that the I427T/Q313R mutant, not only alters NAI susceptibility but also compromises NA properties and viral fitness, which could explain its infrequent detection in clinic.

Platform for determining the inhibition profile of neuraminidase inhibitors in an influenza virus N1 background

Efforts to develop novel neuraminidase inhibitors (NAIs) for the treatment of influenza are ongoing. Novel NAIs should in particular be also effective against seasonal and/or pandemic N1 that carry a H274Y or N294S substitution (N2 numbering), which are most commonly linked to oseltamivir resistance. Here we report a platform for profiling the efficacy of novel NAIs in the N1 genetic background of influenza A virus. Employing reverse genetics, a set of influenza virus variants containing an amino acid substitution associated with oseltamivir resistance in N1 isolates (H274Y, N294S, Y155H or Q136L) was generated. In parallel, so far unreported mutations of I427 (I427Q and I427M) were investigated. These possibly interfere with the side chain orientation of R371 and alter the binding affinity of most relevant NAIs. The profiling platform was validated with both oseltamivir and zanamivir and exemplarily applied to three analogs with differing decorations at positions 4 and 5. Besides confirming the inhibition profile of zanamivir and oseltamivir, the distinct effect of I427Q/M on the activity of both NAIs was shown. For 5-amidino and 5-guanidino analogs of oseltamivir a significantly stronger inhibition of virus variants carrying a NA-H274Y was confirmed, and additionally shown for NA-N294S and NA-Y155H substitutions as compared to the parent compound. Hence, the herein presented profiling platform is a valid tool for defining the inhibition profile of novel NAIs in the N1 background.

Competitive Fitness of Influenza B Viruses Possessing E119A and H274Y Neuraminidase Inhibitor Resistance-Associated Substitutions in Ferrets

Neuraminidase (NA) inhibitors (NAIs) are the only antiviral drugs recommended for influenza treatment and prophylaxis. Although NAI-resistant influenza B viruses that could pose a threat to public health have been reported in the field, their fitness is poorly understood. We evaluated in ferrets the pathogenicity and relative fitness of reverse genetics (rg)-generated influenza B/Yamanashi/166/1998-like viruses containing E119A or H274Y NA substitutions (N2 numbering). Ferrets inoculated with NAI-susceptible rg-wild-type (rg-WT) or NAI-resistant (rg-E119A or rg-H274Y) viruses developed mild infections. Growth of rg-E119A virus in the nasal cavities was delayed, but the high titers at 3 days post-inoculation (dpi) were comparable to those of the rg-WT and rg-H274Y viruses (3.6-4.1 log10TCID50/mL). No virus persisted beyond 5 dpi and replication did not extend to the trachea or lungs. Positive virus antigen-staining of the nasal turbinate epithelium was intermittent with the rg-WT and rg-H274Y viruses; whereas antigen-staining for the rg-E119A virus was more diffuse. Virus populations in ferrets coinoculated with NAI-susceptible and -resistant viruses (1:1 mixture) remained heterogeneous at 5 dpi but were predominantly rg-WT (>70%). Although the E119A substitution was associated with delayed replication in ferrets, the H274Y substitution did not measurably affect viral growth properties. These data suggest that rg-H274Y has undiminished fitness in single virus inoculations, but neither rg-E119A nor rg-H274Y gained a fitness advantage over rg-WT in direct competition experiments without antiviral drug pressure. Taken together, our data suggest the following order of relative fitness in a ferret animal model: rg-WT > rg-H274Y > rg-E119A.

Drug-Resistant and Genetic Evolutionary Analysis of Influenza Virus from Patients During the 2013 and 2014 Influenza Season in Beijing

The study aimed to analyze drug resistance and mutations and genetic evolution of influenza A and influenza B viruses during the 2013 and 2014 influenza season in Beijing, China. RNA was extracted from pharyngeal or nasal swabs of 28 patients, and determination of influenza genotypes was performed by using real-time reverse-transcription polymerase chain reaction. Influenza A virus samples were sequenced with the neuraminidase (NA) gene and M2 matrix protein gene to determine the NA inhibitor (NAI) resistance and amantadine resistance mutations, and influenza B virus samples were sequenced with the NA gene and hemagglutinin (HA) gene to analyze NAI resistance mutations. As a result, the enrolled subjects consisted of 19 patients with the A(H1N1)pdm09 subtype, four with A(H3N2) subtype and five with influenza B virus. All of the 23 samples with influenza A viruses harbored amantadine resistance mutation S31N in M2 matrix protein. V241I, a compensatory NAI resistance mutation, was detected in all of the 19 A(H1N1)pdm09 viruses. No other NAI resistance mutation was observed in both influenza A and B viruses. The NA gene of the five influenza B virus strains was classified as B-Victoria lineage, while the HA gene of five strains was classified as B-Yamagata lineage. In summary, all influenza A viruses from patients in Beijing in the 2013-2014 season were resistant to amantadine agent. Both influenza A and B viruses kept sensitive to NAIs. Lineage recombination was detected in influenza B virus strains and may impair the efficacy of influenza vaccination.

The E119D neuraminidase mutation identified in a multidrug-resistant influenza A(H1N1)pdm09 isolate severely alters viral fitness in vitro and in animal models

We recently isolated an influenza A(H1N1)pdm09 E119D/H275Y neuraminidase (NA) variant from an immunocompromised patient who received oseltamivir and zanamivir therapies. This variant demonstrated cross resistance to zanamivir, oseltamivir, peramivir and laninamivir. In this study, the viral fitness of the recombinant wild-type (WT), E119D and E119D/H275Y A(H1N1)pdm09 viruses was evaluated in vitro and in experimentally-infected C57BL/6 mice and guinea pigs. In replication kinetics experiments, viral titers obtained with the E119D and E119D/H275Y recombinants were up to 2- and 4-log lower compared to the WT virus in MDCK and ST6GalI-MDCK cells, respectively. Enzymatic studies revealed that the E119D mutation significantly decreased the surface NA activity. In experimentally-infected mice, a 50% mortality rate was recorded in the group infected with the WT recombinant virus whereas no mortality was observed in the E119D and E119D/H275Y groups. Mean lung viral titers on day 5 post-inoculation for the WT (1.2 ± 0.57 × 10(8) PFU/ml) were significantly higher than those of the E119D (9.75 ± 0.41 × 10(5) PFU/ml, P < 0.01) and the E119D/H275Y (1.47 ± 0.61 × 10(6) PFU/ml, P < 0.01) groups. In guinea pigs, comparable seroconversion rates and viral titers in nasal washes (NW) were obtained for the WT and mutant index and contact groups. However, the D119E reversion was observed in most NW samples of the E119D and E119D/H275Y animals. In conclusion, the E119D NA mutation that could emerge in A(H1N1)pdm09 viruses during zanamivir therapy has a significant impact on viral fitness and such mutant is unlikely to be highly transmissible.

Characteristics of oseltamivir-resistant influenza A (H1N1) pdm09 virus during the 2013-2014 influenza season in Mainland China

Background

In this study, we analyzed the characteristics of oseltamivir-resistant influenza A (H1N1) pdm09 virus isolated from patients in mainland China during the influenza season from September 2013 through March 2014, and provide guidance on which antiviral to be used for clinical treatment.

Methods

The all viruses collected from September 1, 2013 through March 31, 2014 were obtained from the Chinese National Influenza Surveillance Network. A fluorescence-based assay was used to detect virus sensitivity to neuraminidase inhibitors (NAIs). The hemagglutinin (HA) and neuraminidase (NA) gene of the oseltamivir-resistant viruses were sequenced.

Results

A total of 24 (2.14 %) influenza A (H1N1) pdm09 viruses that were resistant to oseltamivir were identified. These 24 viruses were isolated from 23 patients and no epidemiological link among them could be identified. Except for one virus with the H275H/Y mixture substitution, all the other 23 viruses had H275Y substitution in the NA protein. Sequence analysis revealed that the amino acid substitutions in the HA protein of influenza A (H1N1) pdm09 viruses with H275Y substitution isolated from mainland China were similar to the viruses from clustered cases reported in the United States, and the amino acid substitutions in the NA protein were similar to the viruses reported in Sapporo, Japan in 2013–2014. All of the oseltamivir-resistant viruses in mainland China and Japan possessed additional substitutions N386K, V241I and N369K in the NA protein, while most (>89 %) resistant-viruses from the United States during the same period possess V241I and N369K and did not have the N386K substitution. The N386K substitution was also exist in most sensitive viruses during the same period in mainland China. The amino acid substitutions in both HA and NA protein differed from the clustered cases from Australia reported in 2011 with additional substitutions. The drug-resistant influenza A(H1N1) pdm09 viruses were from patients without any known NAIs medication history prior to sampling.

Conclusions

During the influenza season from September 2013 through March 2014 in Mainland China, oseltamivir-resistant influenza A(H1N1)pdm09 viruses were much more frequently detected than ever since the appearance of the virus in 2009.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0317-1) contains supplementary material, which is available to authorized users.

E119D Neuraminidase Mutation Conferring Pan-Resistance to Neuraminidase Inhibitors in an A(H1N1)pdm09 Isolate From a Stem-Cell Transplant Recipient

BACKGROUND

An influenza A(H1N1)pdm09 infection was diagnosed in a hematopoietic stem cell transplant recipient during conditioning regimen. He was treated with oral oseltamivir, later combined with intravenous zanamivir. The H275Y neuraminidase (NA) mutation was first detected, and an E119D NA mutation was identified during zanamivir therapy.

METHODS

Recombinant wild-type (WT) E119D and E119D/H275Y A(H1N1)pdm09 NA variants were generated by reverse genetics. Susceptibility to NA inhibitors (NAIs) was evaluated with a fluorometric assay using the 2'-(4-methylumbelliferyl)-α-D-N-acetylneuraminic acid (MUNANA) substrate. Susceptibility to favipiravir (T-705) was assessed using plaque reduction assays. The NA affinity and velocity values were determined with NA enzymatic studies.

RESULTS

We identified an influenza A(H1N1)pdm09 E119D mutant that exhibited a marked increase in the 50% inhibitory concentrations against all tested NAIs (827-, 25-, 286-, and 702-fold for zanamivir, oseltamivir, peramivir, and laninamivir, respectively). The double E119D/H275Y mutation further increased oseltamivir and peramivir 50% inhibitory concentrations by 790- and >5000-fold, respectively, compared with the WT. The mutant viruses remained susceptible to favipiravir. The NA affinity and velocity values of the E119D variant decreased by 8.1-fold and 4.5-fold, respectively, compared with the WT.

CONCLUSIONS

The actual emergence of a single NA mutation conferring pan-NAI resistance in the clinical setting reinforces the pressing need to develop new anti-influenza strategies.

Global update on the susceptibility of human influenza viruses to neuraminidase inhibitors, 2013–2014

Four World Health Organization (WHO) Collaborating Centres for Reference and Research on Influenza and one WHO Collaborating Centre for the Surveillance, Epidemiology and Control of Influenza (WHO CCs) tested 10,641 viruses collected by WHO-recognized National Influenza Centres between May 2013 and May 2014 to determine 50% inhibitory concentration (IC₅₀) data for neuraminidase inhibitors (NAIs) oseltamivir, zanamivir, peramivir and laninamivir. In addition, neuraminidase (NA) sequence data, available from the WHO CCs and from sequence databases (n = 3206), were screened for amino acid substitutions associated with reduced NAI susceptibility. Ninety-five per cent of the viruses tested by the WHO CCs were from three WHO regions: Western Pacific, the Americas and Europe. Approximately 2% (n = 172) showed highly reduced inhibition (HRI) against at least one of the four NAIs, commonly oseltamivir, while 0.3% (n = 32) showed reduced inhibition (RI). Those showing HRI were A(H1N1)pdm09 with NA H275Y (n = 169), A(H3N2) with NA E119V (n = 1), B/Victoria-lineage with NA E117G (n = 1) and B/Yamagata-lineage with NA H273Y (n = 1); amino acid position numbering is A subtype and B type specific. Although approximately 98% of circulating viruses tested during the 2013–2014 period were sensitive to all four NAIs, a large community cluster of A(H1N1)pdm09 viruses with the NA H275Y substitution from patients with no previous exposure to antivirals was detected in Hokkaido, Japan. Significant numbers of A(H1N1)pdm09 NA H275Y viruses were also detected in China and the United States: phylogenetic analyses showed that the Chinese viruses were similar to those from Japan, while the United States viruses clustered separately from those of the Hokkaido outbreak, indicative of multiple resistance-emergence events. Consequently, global surveillance of influenza antiviral susceptibility should be continued from a public health perspective.

Characterization of a large cluster of influenza A(H1N1)pdm09 viruses cross-resistant to oseltamivir and peramivir during the 2013-2014 influenza season in Japan

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.

Permissive changes in the neuraminidase play a dominant role in improving the viral fitness of oseltamivir-resistant seasonal influenza A(H1N1) strains

Permissive neuraminidase (NA) substitutions such as R222Q, V234M and D344N have facilitated the emergence and worldwide spread of oseltamivir-resistant influenza A/Brisbane/59/2007 (H1N1)-H275Y viruses. However, the potential contribution of genetic changes in other viral segments on viral fitness remains poorly investigated. A series of recombinant A(H1N1)pdm09 and A/WSN/33 7:1 reassortants containing the wild-type (WT) A/Brisbane/59/2007 NA gene or its single (H275Y) and double (H275Y/Q222R, H275Y/M234V and H275Y/N344D) variants were generated and their replicative properties were assessed in vitro. The Q222R reversion substitution significantly reduced viral titers when evaluated in both A(H1N1)pdm09 and A/WSN/33 backgrounds. The permissive role of the R222Q was further confirmed using A/WSN/33 7:1 reassortants containing the NA gene of the oseltamivir-susceptible or oseltamivir-resistant influenza A/Mississippi/03/2001 strains. Therefore, NA permissive substitutions play a dominant role for improving viral replication of oseltamivir-resistant A (H1N1)-H275Y viruses in vitro.

Epistatic interactions between neuraminidase mutations facilitated the emergence of the oseltamivir-resistant H1N1 influenza viruses

Oseltamivir-resistant H1N1 influenza viruses carrying the H275Y neuraminidase mutation predominated worldwide during the 2007–2009 seasons. While several neuraminidase substitutions were found to be necessary to counteract the adverse effects of H275Y, the order and impact of evolutionary events involved remain elusive. Here, we reconstruct H1N1 neuraminidase phylogeny during 1999–2009, estimate the timing and order of crucial amino acid changes, and evaluate their impact on the biological outcome of the H275Y mutation. Of the twelve neuraminidase substitutions that occurred during 1999–2009, five (chronologically, V234M, R222Q, K329E, D344N, H275Y, and D354G) are necessary for maintaining full neuraminidase function in the presence of the H275Y mutation by altering protein accumulation or enzyme affinity/activity. The sequential emergence and cumulative effects of these mutations clearly illustrate a role for epistasis in shaping the emergence and subsequent evolution of a drug-resistant virus population, which can be useful in understanding emergence of novel viral phenotypes of influenza.

VIRAPOPS2 supports the influenza virus reassortments

BACKGROUND

For over 400 years, due to the reassortment of their segmented genomes, influenza viruses evolve extremely quickly and cause devastating epidemics. This reassortment arises because two flu viruses can infect the same cell and therefore the new virions' genomes will be composed of segment reassortments of the two parental strains. A treatment developed against parents could then be ineffective if the virions' genomes are different enough from their parent's genomes. It is therefore essential to simulate such reassortment phenomena to assess the risk of apparition of new flu strain.

FINDINGS

So we decided to upgrade the forward simulator VIRAPOPS, containing already the necessary options to handle non-segmented viral populations. This new version can mimic single or successive reassortments, in birds, humans and/or swines. Other options such as the ability to treat populations of positive or negative sense viral RNAs, were also added. Finally, we propose output options giving statistics of the results.

CONCLUSION

In this paper we present a new version of VIRAPOPS which now manages the viral segment reassortments and the negative sense single strain RNA viruses, these two issues being the cause of serious public health problems.

Characterization of drug-resistant influenza A(H7N9) variants isolated from an oseltamivir-treated patient in Taiwan

BACKGROUND

Patients contracting influenza A(H7N9) infection often developed severe disease causing respiratory failure. Neuraminidase (NA) inhibitors (NAIs) are the primary option for treatment, but information on drug-resistance markers for influenza A(H7N9) is limited.

METHODS

Four NA variants of A/Taiwan/1/2013(H7N9) virus containing a single substitution (NA-E119V, NA-I222K, NA-I222R, or NA-R292K) recovered from an oseltamivir-treated patient were tested for NAI susceptibility in vitro; their replicative fitness was evaluated in cell culture, mice, and ferrets.

RESULTS

NA-R292K led to highly reduced inhibition by oseltamivir and peramivir, while NA-E119V, NA-I222K, and NA-I222R caused reduced inhibition by oseltamivir. Mice infected with any virus showed severe clinical signs with high mortality rates. NA-I222K virus was the most virulent in mice, whereas virus lacking NA change (NA-WT) and NA-R292K virus seemed the least virulent. Sequence analysis suggests that PB2-S714N increased virulence of NA-I222K virus in mice; NS1-K126R, alone or in combination with PB2-V227M, produced contrasting effects in NA-WT and NA-R292K viruses. In ferrets, all viruses replicated to high titers in the upper respiratory tract but produced only mild illness. NA-R292K virus, showed reduced replicative fitness in this animal model.

CONCLUSIONS

Our data highlight challenges in assessment of the replicative fitness of H7N9 NA variants that emerged in NAI-treated patients.

Evolution of oseltamivir resistance mutations in Influenza A(H1N1) and A(H3N2) viruses during selection in experimentally infected mice

The evolution of oseltamivir resistance mutations during selection through serial passages in animals is still poorly described. Herein, we assessed the evolution of neuraminidase (NA) and hemagglutinin (HA) genes of influenza A/WSN/33 (H1N1) and A/Victoria/3/75 (H3N2) viruses recovered from the lungs of experimentally infected BALB/c mice receiving suboptimal doses (0.05 and 1 mg/kg of body weight/day) of oseltamivir over two generations. The traditional phenotypic and genotypic methods as well as deep-sequencing analysis were used to characterize the potential selection of mutations and population dynamics of oseltamivir-resistant variants. No oseltamivir-resistant NA or HA changes were detected in the recovered A/WSN/33 viruses. However, we observed a positive selection of the I222T NA substitution in the recovered A/Victoria/3/75 viruses, with a frequency increasing over time and with an oseltamivir concentration from 4% in the initial pretherapy inoculum up to 28% after two lung passages. Although the presence of mixed I222T viral populations in mouse lungs only led to a minimal increase in oseltamivir 50% enzyme-inhibitory concentrations (IC50s) (by a mean of 5.7-fold) compared to that of the baseline virus, the expressed recombinant A/Victoria/3/75 I222T NA protein displayed a 16-fold increase in the oseltamivir IC50 level compared to that of the recombinant wild type (WT). In conclusion, the combination of serial in vivo passages under neuraminidase inhibitor (NAI) pressure and temporal deep-sequencing analysis enabled, for the first time, the identification and selection of the oseltamivir-resistant I222T NA mutation in an influenza H3N2 virus. Additional in vivo selection experiments with other antivirals and drug combinations might provide important information on the evolution of antiviral resistance in influenza viruses.

Epitope mapping of the 2009 pandemic and the A/Brisbane/59/2007 seasonal (H1N1) influenza virus haemagglutinins using mAbs and escape mutants

mAbs constitute an important biological tool for influenza virus haemagglutinin (HA) epitope mapping through the generation of escape mutants, which could provide insights into immune evasion mechanisms and may benefit the future development of vaccines. Several influenza A (H1N1) pandemic 2009 (pdm09) HA escape mutants have been recently described. However, the HA antigenic sites of the previous seasonal A/Brisbane/59/2007 (H1N1) (Bris07) virus remain poorly documented. Here, we produced mAbs against pdm09 and Bris07 HA proteins expressed in human HEK293 cells. Escape mutants were generated using mAbs that exhibited HA inhibition and neutralizing activities. The resulting epitope mapping of the pdm09 HA protein revealed 11 escape mutations including three that were previously described (G172E, N173D and K256E) and eight novel ones (T89R, F128L, G157E, K180E, A212E, R269K, N311T and G478E). Among the six HA mutations that were part of predicted antigenic sites (Ca1, Ca2, Cb, Sa or Sb), three (G172E, N173D and K180E) were within the Sa site. Eight escape mutations (H54N, N55D, N55K, L60H, N203D, A231T, V314I and K464E) were obtained for Bris07 HA, and all but one (N203D, Sb site) were outside the predicted antigenic sites. Our results suggest that the Sa antigenic site is immunodominant in pdm09 HA, whereas the N203D mutation (Sb site), present in three different Bris07 escape mutants, appears as the immunodominant epitope in that strain. The fact that some mutations were not part of predicted antigenic sites reinforces the necessity of further characterizing the HA of additional H1N1 strains.

Impact of different oseltamivir regimens on treating influenza A virus infection and resistance emergence: insights from a modelling study

Several studies have proven oseltamivir to be efficient in reducing influenza viral titer and symptom intensity. However, the usefulness of oseltamivir can be compromised by the emergence and spread of drug-resistant virus. The selective pressure exerted by different oseltamivir therapy regimens have received little attention. Combining models of drug pharmacokinetics, pharmacodynamics, viral kinetics and symptom dynamics, we explored the efficacy of oseltamivir in reducing both symptoms (symptom efficacy) and viral load (virological efficacy). We simulated samples of 1000 subjects using previously estimated between-subject variability in viral and symptom dynamic parameters to describe the observed heterogeneity in a patient population. We simulated random mutations conferring resistance to oseltamivir. We explored the effect of therapy initiation time, dose, intake frequency and therapy duration on influenza infection, illness dynamics, and emergence of viral resistance. Symptom and virological efficacies were strongly associated with therapy initiation time. The proportion of subjects shedding resistant virus was 27-fold higher when prophylaxis was initiated during the incubation period compared with no treatment. It fell to below 1% when treatment was initiated after symptom onset for twice-a-day intakes. Lower doses and prophylaxis regimens led to lower efficacies and increased risk of resistance emergence. We conclude that prophylaxis initiated during the incubation period is the main factor leading to resistance emergence.

Oseltamivir is currently the most commonly used drug against influenza but the emergence and spread of oseltamivir-resistant virus is threatening its usefulness. A previously published study quantified the risk of drug-resistance emergence and spread. In this work we investigate under what conditions drug-resistance is likely to occur and how we can mitigate it. For this purpose, we simulated populations of influenza-infected subjects under different treatment conditions varying drug dose, intake frequency and duration of therapy. We used an approach that mimics the randomness of drug-resistance emergence and allowed for between-subject variability. We measured the effect of treatment on reducing infection and symptoms and on drug-resistance emergence. We found that for subjects starting oseltamivir during the influenza incubation period, the risk of resistance emergence is dramatically increased. Thus, our findings suggest that standard prophylaxis should only be used after exclusion of an influenza infection in the incubation period by use of a rapid test. If existing infection cannot be excluded, then prophylaxis should be done with increased dose, intake frequency and duration in order to avoid emergence of drug-resistant strains and to preserve oseltamivir efficacy.

Estimating the fitness advantage conferred by permissive neuraminidase mutations in recent oseltamivir-resistant A(H1N1)pdm09 influenza viruses

Oseltamivir is relied upon worldwide as the drug of choice for the treatment of human influenza infection. Surveillance for oseltamivir resistance is routinely performed to ensure the ongoing efficacy of oseltamivir against circulating viruses. Since the emergence of the pandemic 2009 A(H1N1) influenza virus (A(H1N1)pdm09), the proportion of A(H1N1)pdm09 viruses that are oseltamivir resistant (OR) has generally been low. However, a cluster of OR A(H1N1)pdm09 viruses, encoding the neuraminidase (NA) H275Y oseltamivir resistance mutation, was detected in Australia in 2011 amongst community patients that had not been treated with oseltamivir. Here we combine a competitive mixtures ferret model of influenza infection with a mathematical model to assess the fitness, both within and between hosts, of recent OR A(H1N1)pdm09 viruses. In conjunction with data from in vitro analyses of NA expression and activity we demonstrate that contemporary A(H1N1)pdm09 viruses are now more capable of acquiring H275Y without compromising their fitness, than earlier A(H1N1)pdm09 viruses circulating in 2009. Furthermore, using reverse engineered viruses we demonstrate that a pair of permissive secondary NA mutations, V241I and N369K, confers robust fitness on recent H275Y A(H1N1)pdm09 viruses, which correlated with enhanced surface expression and enzymatic activity of the A(H1N1)pdm09 NA protein. These permissive mutations first emerged in 2010 and are now present in almost all circulating A(H1N1)pdm09 viruses. Our findings suggest that recent A(H1N1)pdm09 viruses are now more permissive to the acquisition of H275Y than earlier A(H1N1)pdm09 viruses, increasing the risk that OR A(H1N1)pdm09 will emerge and spread worldwide.