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Gilbertson B, Duncan M, Subbarao K. Role of the viral polymerase during adaptation of influenza A viruses to new hosts. Curr Opin Virol 2023; 62:101363. [PMID: 37672875 DOI: 10.1016/j.coviro.2023.101363] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2023] [Revised: 08/15/2023] [Accepted: 08/15/2023] [Indexed: 09/08/2023]
Abstract
As a group, influenza-A viruses (IAV) infect a wide range of animal hosts, however, they are constrained to infecting selected host species by species-specific interactions between the host and virus, that are required for efficient replication of the viral RNA genome. When IAV cross the species barrier, they acquire mutations in the viral genome to enable interactions with the new host factors, or to compensate for their loss. The viral polymerase genes polymerase basic 1, polymerase basic 2, and polymerase-acidic are important sites of host adaptation. In this review, we discuss why the viral polymerase is so vital to the process of host adaptation, look at some of the known viral mutations, and host factors involved in adaptation, particularly of avian IAV to mammalian hosts.
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Affiliation(s)
- Brad Gilbertson
- Department of Microbiology and Immunology, University of Melbourne, at The Peter Doherty Institute for Infection and Immunity, Melbourne, Victoria, Australia
| | - Melanie Duncan
- WHO Collaborating Centre for Reference and Research on Influenza, Victorian Infectious Diseases Reference Laboratory at The Peter Doherty Institute for Infection and Immunity, Melbourne, Victoria, Australia
| | - Kanta Subbarao
- Department of Microbiology and Immunology, University of Melbourne, at The Peter Doherty Institute for Infection and Immunity, Melbourne, Victoria, Australia; WHO Collaborating Centre for Reference and Research on Influenza, Victorian Infectious Diseases Reference Laboratory at The Peter Doherty Institute for Infection and Immunity, Melbourne, Victoria, Australia.
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2
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Guan L, Babujee L, Browning VL, Presler R, Pattinson D, Nguyen HLK, Hoang VMP, Le MQ, van Bakel H, Neumann G, Kawaoka Y. Continued Circulation of Highly Pathogenic H5 Influenza Viruses in Vietnamese Live Bird Markets in 2018-2021. Viruses 2023; 15:1596. [PMID: 37515281 PMCID: PMC10384249 DOI: 10.3390/v15071596] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2023] [Revised: 07/18/2023] [Accepted: 07/19/2023] [Indexed: 07/30/2023] Open
Abstract
We isolated 77 highly pathogenic avian influenza viruses during routine surveillance in live poultry markets in northern provinces of Vietnam from 2018 to 2021. These viruses are of the H5N6 subtype and belong to HA clades 2.3.4.4g and 2.3.4.4h. Interestingly, we did not detect viruses of clade 2.3.4.4b, which in recent years have dominated in different parts of the world. The viruses isolated in this current study do not encode major determinants of mammalian adaptation (e.g., PB2-E627K or PB1-D701N) but possess amino acid substitutions that may affect viral receptor-binding, replication, or the responses to human antiviral factors. Several of the highly pathogenic H5N6 virus samples contained other influenza viruses, providing an opportunity for reassortment. Collectively, our study demonstrates that the highly pathogenic H5 viruses circulating in Vietnam in 2018-2021 were different from those in other parts of the world, and that the Vietnamese H5 viruses continue to evolve through mutations and reassortment.
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Affiliation(s)
- Lizheng Guan
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - Lavanya Babujee
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - Victoria L. Browning
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - Robert Presler
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - David Pattinson
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - Hang Le Khanh Nguyen
- National Institute of Hygiene and Epidemiology, Hanoi 100000, Vietnam; (H.L.K.N.); (V.M.P.H.); (M.Q.L.)
| | - Vu Mai Phuong Hoang
- National Institute of Hygiene and Epidemiology, Hanoi 100000, Vietnam; (H.L.K.N.); (V.M.P.H.); (M.Q.L.)
| | - Mai Quynh Le
- National Institute of Hygiene and Epidemiology, Hanoi 100000, Vietnam; (H.L.K.N.); (V.M.P.H.); (M.Q.L.)
| | - Harm van Bakel
- Department of Genetics and Genomic Services, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA;
| | - Gabriele Neumann
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
| | - Yoshihiro Kawaoka
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI 53711, USA; (L.G.); (L.B.); (V.L.B.); (R.P.); (D.P.)
- Division of Virology, Department of Microbiology and Immunology, International Research Center for Infectious Diseases, The Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan
- Research Center for Global Viral Diseases, National Center for Global Health and Medicine, Tokyo 162-8655, Japan
- The University of Tokyo Pandemic Preparedness, Infection and Advanced Research (UTOPIA) Center, Tokyo 108-8639, Japan
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Guan L, Zhong G, Fan S, Plisch EM, Presler R, Gu C, Babujee L, Pattinson D, Le Khanh Nguyen H, Hoang VMP, Le MQ, van Bakel H, Neumann G, Kawaoka Y. Highly Pathogenic H5 Influenza Viruses Isolated between 2016 and 2017 in Vietnamese Live Bird Markets. Viruses 2023; 15:1093. [PMID: 37243179 PMCID: PMC10223276 DOI: 10.3390/v15051093] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2023] [Revised: 04/21/2023] [Accepted: 04/21/2023] [Indexed: 05/28/2023] Open
Abstract
Routine surveillance in live poultry markets in the northern regions of Vietnam from 2016 to 2017 resulted in the isolation of 27 highly pathogenic avian H5N1 and H5N6 viruses of 3 different clades (2.3.2.1c, 2.3.4.4f, and 2.3.4.4g). Sequence and phylogenetic analysis of these viruses revealed reassortment with various subtypes of low pathogenic avian influenza viruses. Deep-sequencing identified minor viral subpopulations encoding variants that may affect pathogenicity and sensitivity to antiviral drugs. Interestingly, mice infected with two different clade 2.3.2.1c viruses lost body weight rapidly and succumbed to virus infection, whereas mice infected with clade 2.3.4.4f or 2.3.4.4g viruses experienced non-lethal infections.
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Affiliation(s)
- Lizheng Guan
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Gongxun Zhong
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Shufang Fan
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Erin M. Plisch
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Robert Presler
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Chunyang Gu
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Lavanya Babujee
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - David Pattinson
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | | | | | - Mai Quynh Le
- National Institute of Hygiene and Epidemiology, Hanoi 100000, Vietnam
| | - Harm van Bakel
- Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA
| | - Gabriele Neumann
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
| | - Yoshihiro Kawaoka
- Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53711, USA
- Division of Virology, Department of Microbiology and Immunology, and International Research Center for Infectious Diseases, The Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan
- Research Center for Global Viral Diseases, National Center for Global Health and Medicine, Tokyo 162-8655, Japan
- Infection and Advanced Research (UTOPIA) Center, The University of Tokyo, Pandemic Preparedness, Tokyo 108-8639, Japan
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Larbi I, Ghedira K, Arbi M, Butcher GD, Rego N, Naya H, Tougorti H, Lachhab J, Behi IE, Nsiri J, Ghram A. Phylogenetic analysis and assessment of the pathogenic potential of the first H9N2 avian influenza viruses isolated from wild birds and Lagoon water in Tunisia. Virus Res 2022; 322:198929. [PMID: 36126884 DOI: 10.1016/j.virusres.2022.198929] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2022] [Revised: 09/01/2022] [Accepted: 09/14/2022] [Indexed: 12/24/2022]
Abstract
H9N2 avian influenza virus (AIV) has been isolated from various species of wild birds and domestic poultry worldwide. It has been reported since the late 1990s, that H9N2 AIV has infected humans as reported in some Asian and North African countries. This subtype has already been circulating and constituting a serious threat to the poultry industry in Tunisia back in 2009. To investigate zoonotic potential and pathogenicity of H9N2 AIV in chickens and mice in Tunisia, five strains have been isolated during the period from 2014 to 2018. Samples were withdrawn from several wild bird species and environment (Lagoon water) of Maamoura and Korba Lagoons as well as Kuriat Island. Phylogenetic analyzes demonstrated that the isolated H9N2 strains belonged to the G1-like sublineage and were close to AIV H9N2 poultry viruses from North Africa, West Africa and the Middle East. All strains carried in their hemagglutinin the residue 226 L, which is an important marker for avian-to-human viral transmission. The hemagglutinin cleavage site has several motifs: PSKSSR/G, PARSSR/G and HARSSR/G. The neuraminidase showed S372A and R403W substitutions that have been previously detected in H3N2 and H2N2 viruses that were reported in human pandemics. Many mutations associated with mammalian infections have been detected in internal proteins. Pathogenicity evaluation in chickens showed that GF/14 replicates effectively in the lungs, tracheas, spleens, kidneys and brains and that it was transmitted among contact chickens. However, GHG/18 replicates poorly in chickens and has not an efficient transmission in contact chickens. GF/14 and GHG/18 could not kill mice though they replicated in their respiratory tract and caused a significant body weight loss (p < 0.05). This study highlights the importance of H9N2 AIV monitoring in both migratory birds and the environment to prevent virus transmission to humans.
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Affiliation(s)
- Imen Larbi
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia.
| | - Kais Ghedira
- Laboratory of Bioinformatics, Biomathematics and Biostatistics, LR16IPT09, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Marwa Arbi
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Gary David Butcher
- College of Veterinary Medicine, University of Florida, Gainesville, FL, United States
| | - Natalia Rego
- Bioinformatics Unit, Institut Pasteur de Montevideo, Mataojo, Montevideo, Uruguay
| | - Hugo Naya
- Bioinformatics Unit, Institut Pasteur de Montevideo, Mataojo, Montevideo, Uruguay; Departmento de Producción Animal y Pasturas, Facultad de Agronomía, Universidad de la República, Uruguay
| | - Halima Tougorti
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Jihene Lachhab
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Imen El Behi
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Jihene Nsiri
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
| | - Abdeljelil Ghram
- Laboratory of Epidemiology and Veterinary Microbiology, LR19IPT03, Institut Pasteur de Tunis, University of Tunis El Manar, Tunis, Tunisia
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Zhu J, Jiang Z, Liu J. The matrix gene of pdm/09 H1N1 contributes to the pathogenicity and transmissibility of SIV in mammals. Vet Microbiol 2021; 255:109039. [PMID: 33740730 DOI: 10.1016/j.vetmic.2021.109039] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2020] [Accepted: 03/10/2021] [Indexed: 01/07/2023]
Abstract
The H1N1 influenza virus of swine-origin was responsible for the H1N1 pandemic in 2009 (pdm/09 H1N1), where the virus was transmitted to humans and then spread between people, and its continued circulation has resulted in it becoming a seasonal human flu virus. Since 2016, the matrix (M) gene of pdm/09 H1N1 has been involved in the reassortment of swine influenza viruses (SIVs) in China and has gradually become a dominant genotype in pigs. However, whether M gene substitution will influence the fitness of emerging SIVs remains unclear. Here, we analyzed the biological characteristics of SIVs with the M gene from Eurasian avian-like (EA) SIV or pdm/09 H1N1 in mammals and found that SIVs containing the pdm/09-M gene exhibit stronger virulence in mice, more efficient respiratory droplet transmission between ferrets, and increased transcription of viral genes in A549 cells compared with those containing EA-M. We also determined the functional significance of the pdm/09-M gene in conferring an elevated release of progeny viruses comprised of largely filamentous virions rather than spherical virions. Our study suggests that pdm/09-M plays a crucial role in the genesis of emerging SIVs in terms of the potential prevalence in the population.
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Affiliation(s)
- Junda Zhu
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, College of Veterinary Medicine and State Key Laboratory of Agrobiotechnology, China Agricultural University, Beijing, 100094, China
| | - Zhimin Jiang
- Chinese Academy of Sciences Key Laboratory of Infection and Immunity, Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Road, Chaoyang District, Beijing, 100101, China
| | - Jinhua Liu
- Key Laboratory of Animal Epidemiology of the Ministry of Agriculture, College of Veterinary Medicine and State Key Laboratory of Agrobiotechnology, China Agricultural University, Beijing, 100094, China.
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Tian J, Qu N, Jiao X, Wang X, Geng J, Griffin N, Shan F. Methionine enkephalin inhibits influenza A virus infection through upregulating antiviral state in RAW264.7 cells. Int Immunopharmacol 2019; 78:106032. [PMID: 31835089 DOI: 10.1016/j.intimp.2019.106032] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2019] [Revised: 11/04/2019] [Accepted: 11/05/2019] [Indexed: 01/27/2023]
Abstract
MENK, as an immune adjuvant, has potential immune-regulatory activity on innate and adaptive immune cells. The aim of this work was to investigate the antiviral effect of MENK on influenza virus-infected murine macrophage cells (RAW264.7) and its underlying mechanisms. The results showed that MENK markedly inhibited influenza A virus (H1N1) replication in pre- and post-MENK treatment, especially in pre-MENK treatment. The mechanisms exploration revealed that MENK (10 mg/mL) significantly inhibited the nucleoprotein (NP) of influenza virus and up-regulated levels of IL-6, TNF-α and IFN-β compared with those in H1N1 control group. Further experiments confirmed that antiviral effects of MENK was associated with promotion of opioid receptor (MOR) as well as activation of NF-κB p65 inducing cellular antiviral status. The data suggest that MENK should be potential candidate for prophylactic or therapeutic treatment against H1N1 influenza virus.
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Affiliation(s)
- Jing Tian
- Department of Immunology, School of Basic Medical Science, Jinzhou Medical University, Jinzhou 121001, China; Department of Immunology, School of Basic Medical Science, China Medical University, Shenyang 110122, China
| | - Na Qu
- Department of Gynecology, Cancer Hospital, China Medical University, Shenyang 110042, China
| | - Xue Jiao
- Department of Translational Medicine, No.4 Teaching Hospital, China Medical University, Shenyang 110032, China
| | - Xiaonan Wang
- Department of Immunology, School of Basic Medical Science, China Medical University, Shenyang 110122, China
| | - Jin Geng
- Department of Ophthalmology, No.1 Teaching Hospital, China Medical University, Shenyang 110001, China
| | - Noreen Griffin
- Immune Therapeutics, Inc., 37 North Orange Avenue, Suite 607, Orlando, FL 32801, USA
| | - Fengping Shan
- Department of Immunology, School of Basic Medical Science, China Medical University, Shenyang 110122, China.
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Zhong G, Fan S, Lopes TJS, Le MQ, van Bakel H, Dutta J, Smith GJD, Jayakumar J, Nguyen HLK, Hoang PVM, Halfmann P, Hatta M, Su YCF, Neumann G, Kawaoka Y. Isolation of Highly Pathogenic H5N1 Influenza Viruses in 2009-2013 in Vietnam. Front Microbiol 2019; 10:1411. [PMID: 31293548 PMCID: PMC6603144 DOI: 10.3389/fmicb.2019.01411] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2018] [Accepted: 06/05/2019] [Indexed: 11/13/2022] Open
Abstract
Routine surveillance and surveillance in response to influenza outbreaks in avian species in Vietnam in 2009-2013 resulted in the isolation of numerous H5N1 influenza viruses of clades 1.1.2, 2.3.2.1a, 2.3.2.1b, 2.3.2.1c, and 2.3.4.1. Consistent with other studies, we found that viruses of clade 2.3.2.1c were dominant in Vietnam in 2013 and circulated in the northern, central, and southern parts of the country. Phylogenetic analysis revealed reassortment among viruses of clades 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c; in contrast, no reassortment was detected between clade 2.3.2.1 viruses and viruses of clades 1.1.2 or 2.3.4.1, respectively. Deep-sequencing of 42 of the 53 isolated H5N1 viruses revealed viral subpopulations encoding variants that may affect virulence, host range, or sensitivity to antiviral compounds; virus isolates containing these subpopulations may have a higher potential to transmit and adapt to mammals. Among the viruses sequenced, a relatively high number of non-synonymous nucleotide polymorphisms was detected in a virus isolated from a barn swallow, possibly suggesting influenza virus adaption to this host.
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Affiliation(s)
- Gongxun Zhong
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | - Shufang Fan
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | - Tiago J S Lopes
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | - Mai Quynh Le
- National Institute of Hygiene and Epidemiology, Hanoi, Vietnam
| | - Harm van Bakel
- Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, NY, United States
| | - Jayeeta Dutta
- Department of Genetics and Genomic Sciences, Icahn School of Medicine at Mount Sinai, New York, NY, United States
| | - Gavin J D Smith
- Duke-NUS Medical School, Singapore, Singapore.,Duke Global Health Institute, Duke University, Durham, NC, United States
| | | | | | | | - Peter Halfmann
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | - Masato Hatta
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | | | - Gabriele Neumann
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States
| | - Yoshihiro Kawaoka
- Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI, United States.,Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo, Japan
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Mutations in the PA Protein of Avian H5N1 Influenza Viruses Affect Polymerase Activity and Mouse Virulence. J Virol 2018; 92:JVI.01557-17. [PMID: 29212927 DOI: 10.1128/jvi.01557-17] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2017] [Accepted: 11/28/2017] [Indexed: 01/09/2023] Open
Abstract
To study the influenza virus determinants of pathogenicity, we characterized two highly pathogenic avian H5N1 influenza viruses isolated in Vietnam in 2012 (A/duck/Vietnam/QT1480/2012 [QT1480]) and 2013 (A/duck/Vietnam/QT1728/2013 [QT1728]) and found that the activity of their polymerase complexes differed significantly, even though both viruses were highly pathogenic in mice. Further studies revealed that the PA-S343A/E347D (PA with the S-to-A change at position 343 and the E-to-D change at position 347) mutations reduced viral polymerase activity and mouse virulence when tested in the genetic background of QT1728 virus. In contrast, the PA-343S/347E mutations increased the polymerase activity of QT1480 and the virulence of a low-pathogenic H5N1 influenza virus. The PA-343S residue (which alone increased viral polymerase activity and mouse virulence significantly relative to viral replication complexes encoding PA-343A) is frequently found in H5N1 influenza viruses of several subclades; infection with a virus possessing this amino acid may pose an increased risk to humans.IMPORTANCE H5N1 influenza viruses cause severe infections in humans with a case fatality rate that exceeds 50%. The factors that determine the high virulence of these viruses in humans are not fully understood. Here, we identified two amino acid changes in the viral polymerase PA protein that affect the activity of the viral polymerase complex and virulence in mice. Infection with viruses possessing these amino acid changes may pose an increased risk to humans.
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