禽流感病毒NS第263～277位核苷酸缺失降低其抗干扰素能力

2000年以来,多数H5N1亚型禽流感病毒在NS基因的263～277位发牛15个碱基的缺失。为了研究此缺失在流感病毒进化中的生物学意义,构建H5N1亚型流感病毒A／SD／04株的HA、NA、NS的全基因表达载体,以及NS基因263～277位删除的突变载体。通过反向遗传学技术,与编码WSN的其他内部基因（PB2,PB1,PA,NP和M）的表达载体进行组合转染,获得在NS基因的263～277位缺失和不缺失的2个重组H5N1亚型流感病毒（RWSN—m248和RWSN-248）。此两个重组病毒在无干扰素产生的Vero细胞上的繁殖滴度相似,在能产生干扰素的细胞MDCK和COS-1细胞上的繁殖滴度有明显差异。两个重组病毒在鸡胚中的繁殖滴,IVPI,MDT和EID50均无显著差异。说明NS基因的263～277位核苷酸的缺失不影响病毒的整体毒力,但降低了H5N1的抗干扰素能力。     

H3N2亚型猪流感病毒的拯救及HA与NA基因的替换

摘要：【目的】为研究流感病毒突破种间屏障分子机制,筛选流感基因工程疫苗株。【方法】本实验以猪流感病毒A/Swine/Henan/S4/01(H3N2)为亲本株,利用反向遗传学操作技术,采用RT-PCR技术对该病毒的8个基因片段分段进行扩增,通过与双向转录载体pHW2000连接, 重组质粒转染293T和MDCK共培养细胞,拯救出全部基因均来自于亲本株的猪流感病毒rgH3N2,并分别以人流感病毒A/ PR/8/34(H1N1)、禽流感病毒A/Duck/Nanchang /4-165/2000 (H4N6 )、马流感病毒A/Equine/Fuyun/2008/(H3N8)的HA和NA基因替换A/Swine/Henan/S4/01的相应基因,【结果】生物学实验结果表明rgH3N2在鸡胚半数感染量、组织培养半数感染量、稳定性试验等方面都与亲本株保持一致。rgH3N2经鸡胚多次传代后血凝价最高可达到1:256,接种MDCK细胞60 h后,血凝价可以达到1:64。基因替换后成功拯救出的重组病毒rgH1N1、rgH4N6和rgH3N8在鸡胚和细胞上均具有较高的增殖能力。【结论】病毒的成功拯救为流感病毒突破种间屏障分子机制,HA、NA基因在流感病毒跨种属传播中所扮演角色的研究和流感基因工程疫苗株的筛选奠定了基础。     

重组细胞高产型H3N2猪流感病毒株的拯救

为拯救出一株能够在动物传代细胞中高水平复制的H3N2亚型猪流感疫苗株,利用反向遗传操作技术,将A/Goose/Dalian/3/01(H9N2)毒株的PB1、PA、NP、M、NS基因和A/PR/8/34毒株的PB2基因作为内部基因与猪流感病毒A/Swine/Henan/S4/01(H3N2)毒株的HA、NA基因进行重组,成功拯救出了具有高度细胞适应性毒株rH3N2株,该毒株接毒MDCK细胞60h后,血凝价可以达到1∶512,表明该毒株具有高度适应细胞繁殖特性,为H3N2亚型猪流感病毒细胞培养型疫苗的研制奠定了基础。     

2006～2007年流行流感病毒减毒疫苗株的构建

选择冷适应、温度敏感、减毒的A/Ann Arbor/6/60(H2N2)流感病毒株作为骨架病毒,对其6个内部基因片段进行了全基因合成,同时人工引入5个氨基酸突变(PB1-391E,581G,661T,PB2-265S,NP-34G).HA和NA来源于2006-2007当年流行株A/New Caledonia/20/99(H1N1).8个基因片段通过与改造后的转录载体pAD3000连接,构建8个基因的拯救载体,经测序获得序列准确的拯救质粒:pMDV-A-PB2、pMDV-A-PB1、pMDV-A-PA、pMDV-A-NP、pMDV-A-M、pMDV-A-NS、pMDV-A-HA、pMDV-A-NA.6质粒与当年流行株的表面基因HA和NA进行"6 2"组合的病毒拯救,8个重组质粒共转染COS-1细胞,成功拯救出了具有血凝性的冷适应减毒的重组A型人流感病毒.鸡胚尿囊液中重组病毒的血凝效价为l:279-1:210.构建的A/AA/6/60 6个内部基因的病毒骨架拯救系统,为深入研究冷适应减毒人流感病毒的基因功能和新型疫苗研发奠定了基础.     

禽流感病毒NS第263~277位核苷酸缺失降低其抗干扰素能力*

2000年以来,多数H5N1亚型禽流感病毒在NS基因的263~277位发生15个碱基的缺失。为了研究此缺失在流感病毒进化中的生物学意义,构建H5N1亚型流感病毒A/SD/04株的HA、NA、NS的全基因表达载体,以及NS基因263~277位删除的突变载体。通过反向遗传学技术,与编码WSN的其他内部基因(PB2,PB1,PA,NP和M)的表达载体进行组合转染,获得在NS基因的263~277位缺失和不缺失的2个重组H5N1亚型流感病毒(RWSN-m248和RWSN-248)。此两个重组病毒在无干扰素产生的Ve     

H5N1亚型禽流感病毒NS第263～277位核苷酸缺失提高病毒对鸡的致病力

为研究2000年以来绝大多数H5N1亚型禽流感病毒分离株在非结构基因的第263~277位发生15个碱基缺失现象的生物学意义,构建H5N1 A/D/SD/04株HA、NA、NS的全基因表达/转录载体,以及NS的删除突变载体(m248),A/D/YZ/04株的NS基因表达/转录载体(848)和其补加15个核苷酸的NS突变载体(m848)。构建的载体分别与编码WSN(H1N1)内部基因载体进行组合转染,拯救获得4个具不同NS的重组的H5N1亚型流感病毒:RWSN-848和RWSN-m248在263~277位缺失15个碱基,RWSN-m848和RWSN-248则在相同位置不发生缺失。4个重组病毒的平均鸡胚繁殖效价(HA)、鸡胚的平均死亡时间(MDT)和鸡胚半数感染量(EID50)均无显著差异;但RWSN-848和RWSN-m248对6周龄SPF鸡的致病力明显高于RWSN-m848和RWSN-248。结果说明H5N1的NS基因在263~277位核苷酸发生缺失后,不影响重组H5N1在鸡胚中的繁殖性能,但提高了病毒对鸡的致病力。     

H5N1亚型禽流感病毒NS第263—277位核苷酸缺失提高病毒对鸡的致病力

为研究2000年以来绝大多数H5N1亚型禽流感病毒分离株在非结构基因的第263—277位发生15个碱基缺失现象的生物学意义,构建H5N1A／D／SD／04株HA、NA、NS的全基因表达／转录载体,以及NS的删除突变载体（m248）,A/D/YZ/04株的NS基因表达／转录载体（848）和其补加15个核苷酸的NS突变载体（m848）。构建的载体分别与编码WSN（H1N1）内部基因载体进行组合转染,拯救获得4个具不同NS的重组的H5N1亚型流感病毒：RWSN-848和RWSN—m248在263-277位缺失15个碱基。RWSN-m848和RWSN-248则在相同位置不发生缺失。4个重组病毒的平均鸡胚繁殖效价（HA）、鸡胚的平均死亡时间（MDT）和鸡胚半数感染量（EID50）均无显著差异;但RWSN-848和RWSN-m248对6周龄SPF鸡的致病力明显高于RWSN—m848和RWSN-248。结果说明H5N1的NS基因在263～277位核苷酸发生缺失后,不影响重组H5N1在鸡胚中的繁殖性能,但提高了病毒对鸡的致病力。     

H9N2禽流感病毒反向遗传系统转录／表达载体的构建和验证

设计带有BsmBI、BsaI或AarI酶切位点的引物,用RT-PCR扩增H9N2亚型禽流感病毒(AIV)的8个基因全长片段,克隆入双向转录/表达载体pHW2000,并在PB2、PB1和NA基因中共引入了3个沉默突变标签。将其2个表面基因(HA和NA基因)加上任意1个内部基因,而其它5个内部基因来自A/WSN/33,进行了6种3 5组合形式的基因重排,把相应组合的转录/表达质粒共转染COS-1细胞,均产生了预期组合、有感染性的H9N2亚型流感病毒,表明亲缘关系遥远的流感病毒可以互相获取基因片段产生重组病毒,提示表面结构基因和单个内部基因不足以限制H9N2AIV在哺乳动物细胞上的宿主范围,同时也验证了构建的8个转录/表达载体均能有效工作,为进一步研究H9N2亚型AIV基因结构与功能、AIV与宿主之间的关系打下了基础。     

H9N2禽流感病毒反向遗传系统转录/表达载体的构建和验证

设计带有BsmBI、BsaⅠ或AarⅠ酶切位点的引物,用RT PCR扩增H9N2亚型禽流感病毒(AIV)的8个基因全长片段,克隆入双向转录/表达载体pHW2000,并在PB2、PB1和NA基因中共引入了3个沉默突变标签.将其2个表面基因(HA和NA基因)加上任意1个内部基因,而其它5个内部基因来自A/WSN/33,进行了6种3+5组合形式的基因重排,把相应组合的转录/表达质粒共转染COS-1细胞,均产生了预期组合、有感染性的H9N2亚型流感病毒,表明亲缘关系遥远的流感病毒可以互相获取基因片段产生重组病毒,提示表面结构基因和单个内部基因不足以限制H9N2 AIV在哺乳动物细胞上的宿主范围,同时也验证了构建的8个转录/表达载体均能有效工作,为进一步研究H9N2亚型AIV基因结构与功能、AIV与宿主之间的关系打下了基础.     

华东地区家鸭中N1亚型禽流感病毒NA基因的遗传进化分析

为了解华东地区家鸭内禽流感病毒的遗传进化情况,对2002~2006年分离自华东地区家鸭的3种主要N1亚型的禽流感病毒:2株H1N1、10株H3N1和14株H5N1,共26株病毒的NA基因进行了遗传进化分析。结果表明,华东地区家鸭中的N1亚型的禽流感病毒正处于不断进化状态中。14株H5N1禽流感病毒均在NA的茎部缺失20个氨基酸(49~68位),而其他N1亚型的禽流感病毒的NA都未见发生此缺失。H3N1病毒可能与H1N1病毒发生了NA基因的重排,但是目前还没有直接证据表明华东地区家鸭中H5N1禽流感参与了基因重排。     

The Special Neuraminidase Stalk-Motif Responsible for Increased Virulence and Pathogenesis of H5N1 Influenza A Virus

The variation of highly pathogenic avian influenza H5N1 virus results in gradually increased virulence in poultry, and human cases continue to accumulate. The neuraminidase (NA) stalk region of influenza virus varies considerably and may associate with its virulence. The NA stalk region of all N1 subtype influenza A viruses can be divided into six different stalk-motifs, H5N1/2004-like (NA-wt), WSN-like, H5N1/97-like, PR/8-like, H7N1/99-like and H5N1/96-like. The NA-wt is a special NA stalk-motif which was first observed in H5N1 influenza virus in 2000, with a 20-amino acid deletion in the 49^th to 68^th positions of the stalk region. Here we show that there is a gradual increase of the special NA stalk-motif in H5N1 isolates from 2000 to 2007, and notably, the special stalk-motif is observed in all 173 H5N1 human isolates from 2004 to 2007. The recombinant H5N1 virus with the special stalk-motif possesses the highest virulence and pathogenicity in chicken and mice, while the recombinant viruses with the other stalk-motifs display attenuated phenotype. This indicates that the special stalk-motif has contributed to the high virulence and pathogenicity of H5N1 isolates since 2000. The gradually increasing emergence of the special NA stalk-motif in H5N1 isolates, especially in human isolates, deserves attention by all.     

Biologic importance of neuraminidase stalk length in influenza A virus.

To investigate the biologic importance of the neuraminidase (NA) stalk of influenza A virus, we generated mutant viruses of A/WSN/33 (H1N1) with stalks of various lengths (0 to 52 amino acids), by using the recently developed reverse genetics system. These mutant viruses, including one that lacked the entire stalk, replicated in tissue culture to the level of the parent virus, whose NA stalk contains 24 amino acid residues. In eggs, however, the length of the stalk was correlated with the efficiency of virus replication: the longer the stalk, the better the replication. This finding indicates that the length of the NA stalk affects the host range of influenza A viruses. The NA stalkless mutant was highly attenuated in mice; none of the animals died even after intranasal inoculation of 10(6) PFU of the virus (the dose of the parent virus required to kill 50% of mice was 10(2.5) PFU). Moreover, the stalkless mutant replicated only in the respiratory organs, whereas the parent virus caused systemic infection in mice. Thus, attenuation of the virus with the deletion of the entire NA stalk raises the possibility of its use as live vaccines.     

The Surface Glycoproteins of H5 Influenza Viruses Isolated from Humans, Chickens, and Wild Aquatic Birds Have Distinguishable Properties

In 1997, 18 confirmed cases of human influenza arising from multiple independent transmissions of H5N1 viruses from infected chickens were reported from Hong Kong. To identify possible phenotypic changes in the hemagglutinin (HA) and neuraminidase (NA) of the H5 viruses during interspecies transfer, we compared the receptor-binding properties and NA activities of the human and chicken H5N1 isolates from Hong Kong and of H5N3 and H5N1 viruses from wild aquatic birds. All H5N1 viruses, including the human isolate bound to Sia2-3Gal-containing receptors but not to Sia2-6Gal-containing receptors. This finding formally demonstrates for the first time that receptor specificity of avian influenza viruses may not restrict initial avian-to-human transmission. The H5N1 chicken viruses differed from H5 viruses of wild aquatic birds by a 19-amino-acid deletion in the stalk of the NA and the presence of a carbohydrate at the globular head of the HA. We found that a deletion in the NA decreased its ability to release the virus from cells, whereas carbohydrate at the HA head decreased the affinity of the virus for cell receptors. Comparison of amino acid sequences from GenBank of the HAs and NAs from different avian species revealed that additional glycosylation of the HA and a shortened NA stalk are characteristic features of the H5 and H7 chicken viruses. This finding indicates that changes in both HA and NA may be required for the adaptation of influenza viruses from wild aquatic birds to domestic chickens and raises the possibility that chickens may be a possible intermediate host in zoonotic transmission.     

A 20-Amino-Acid Deletion in the Neuraminidase Stalk and a Five-Amino-Acid Deletion in the NS1 Protein Both Contribute to the Pathogenicity of H5N1 Avian Influenza Viruses in Mallard Ducks

Since 2003, H5N1-subtype avian influenza viruses (AIVs) with both a deletion of 20 amino acids in the stalk of the neuraminidase (NA) glycoprotein (A−) and a deletion of five amino acids at positions 80 to 84 in the non-structural protein NS1 (S−) have become predominant. To understand the influence of these double deletions in the NA and NS1 proteins on the pathogenicity of H5N1-subtype AIVs, we selected A/mallard/Huadong/S/2005 as a parental strain to generate rescued wild-type A−S− and three variants (A−S+ with a five-amino-acid insertion in the NS1 protein, A+S− with a 20-amino-acid insertion in the NA stalk, and A+S+ with insertions in both NA and NS1 proteins) and evaluated their biological characteristics and virulence. The titers of the AIVs with A− and/or S− replicated in DEF cells were higher than that of A+S+, and the A−S− virus exhibited a replication predominance when co-infected with the other variants in DEF cells. In addition, A−S− induced a more significant increase in the expression of immune-related genes in peripheral blood mononuclear cells of mallard ducks in vitro compared with the other variants. Furthermore, an insertion in the NA and/or NS1 proteins of AIVs resulted in a notable decrease in virulence in ducks, as determined by intravenous pathogenicity index, and the two insertions exerted a synergistic effect on the attenuation of pathogenicity in ducks. In addition, compared with A+S+ and A+S−, the A−S+ and A−S− viruses that were introduced via the intranasal inoculation route exhibited a faster replication ability in the lungs of ducks. These data indicate that both the deletions in the NA stalk and the NS1 protein contribute to the high pathogenicity of H5N1 AIVs in ducks.     

H5N1 influenza marker vaccine for serological differentiation between vaccinated and infected chickens

Using plasmid-based reverse genetics, we generated a molecularly altered virus, H5N1/PR8-5B19, containing modified HA and NA genes from A/Goose/Guangdong/1/96 (GS/GD/1/96). In the H5N1/PR8-5B19 virus, the HA cleavage site was modified to resemble that of low-pathogenic avian strains and a portion of the NA stalk region was replaced by the immunodominant 5B19 epitope of the S2 glycoprotein of murine hepatitis virus (MHV). H5N1/PR8-5B19 is not lethal to embryonated eggs or chickens. Chickens immunized with the H5N1/PR8-5B19 inactivated vaccine produced high levels of HI antibody and a measurable antibody response against the MHV 5B19 epitope, and were fully protected against subsequent challenge with different highly pathogenic H5N1 avian influenza viruses. H5N1/PR8-5B19 is therefore an attractive marker vaccine candidate, eliciting a strong, protective antibody response and enabling serological discrimination between vaccinated and wild-type virus-infected chickens.     

The low-pH stability discovered in neuraminidase of 1918 pandemic influenza A virus enhances virus replication

The "Spanish" pandemic influenza A virus, which killed more than 20 million worldwide in 1918-19, is one of the serious pathogens in recorded history. Characterization of the 1918 pandemic virus reconstructed by reverse genetics showed that PB1, hemagglutinin (HA), and neuraminidase (NA) genes contributed to the viral replication and virulence of the 1918 pandemic influenza virus. However, the function of the NA gene has remained unknown. Here we show that the avian-like low-pH stability of sialidase activity discovered in the 1918 pandemic virus NA contributes to the viral replication efficiency. We found that deletion of Thr at position 435 or deletion of Gly at position 455 in the 1918 pandemic virus NA was related to the low-pH stability of the sialidase activity in the 1918 pandemic virus NA by comparison with the sequences of other human N1 NAs and sialidase activity of chimeric constructs. Both amino acids were located in or near the amino acid resides that were important for stabilization of the native tetramer structure in a low-pH condition like the N2 NAs of pandemic viruses that emerged in 1957 and 1968. Two reverse-genetic viruses were generated from a genetic background of A/WSN/33 (H1N1) that included low-pH-unstable N1 NA from A/USSR/92/77 (H1N1) and its counterpart N1 NA in which sialidase activity was converted to a low-pH-stable property by a deletion and substitutions of two amino acid residues at position 435 and 455 related to the low-pH stability of the sialidase activity in 1918 NA. The mutant virus that included "Spanish Flu"-like low-pH-stable NA showed remarkable replication in comparison with the mutant virus that included low-pH-unstable N1 NA. Our results suggest that the avian-like low-pH stability of sialidase activity in the 1918 pandemic virus NA contributes to the viral replication efficiency.     

Neuraminidase inhibitor-resistant recombinant A/Vietnam/1203/04 (H5N1) influenza viruses retain their replication efficiency and pathogenicity in vitro and in vivo

Effective antiviral drugs are essential for early control of an influenza pandemic. It is therefore crucial to evaluate the possible threat posed by neuraminidase (NA) inhibitor-resistant influenza viruses with pandemic potential. Four NA mutations (E119G, H274Y, R292K, and N294S) that have been reported to confer resistance to NA inhibitors were each introduced into recombinant A/Vietnam/1203/04 (VN1203) H5N1 influenza virus. For comparison, the same mutations were introduced into recombinant A/Puerto Rico/8/34 (PR8) H1N1 influenza virus. The E119G and R292K mutations significantly compromised viral growth in vitro, but the H274Y and N294S mutations were stably maintained in VN1203 and PR8 viruses. In both backgrounds, the H274Y and N294S mutations conferred resistance to oseltamivir carboxylate (50% inhibitory concentration [IC(50)] increases, >250-fold and >20-fold, respectively), and the N294S mutation reduced susceptibility to zanamivir (IC(50) increase, >3.0-fold). Although the H274Y and N294S mutations did not compromise the replication efficiency of VN1203 or PR8 viruses in vitro, these mutations slightly reduced the lethality of PR8 virus in mice. However, the VN1203 virus carrying either the H274Y or N294S mutation exhibited lethality similar to that of the wild-type VN1203 virus. The different enzyme kinetic parameters (V(max) and K(m)) of avian-like VN1203 NA and human-like PR8 NA suggest that resistance-associated NA mutations can cause different levels of functional loss in NA glycoproteins of the same subtype. Our results suggest that NA inhibitor-resistant H5N1 variants may retain the high pathogenicity of the wild-type virus in mammalian species. Patients receiving NA inhibitors for H5N1 influenza virus infection should be closely monitored for the emergence of resistant variants.     

NA Proteins of Influenza A Viruses H1N1/2009, H5N1, and H9N2 Show Differential Effects on Infection Initiation,Virus Release,and Cell-Cell Fusion

Two surface glycoproteins of influenza virus, haemagglutinin (HA) and neuraminidase (NA), play opposite roles in terms of their interaction with host sialic acid receptors. HA attaches to sialic acid on host cell surface receptors to initiate virus infection while NA removes these sialic acids to facilitate release of progeny virions. This functional opposition requires a balance. To explore what might happen when NA of an influenza virus was replaced by one from another isolate or subtype, in this study, we generated three recombinant influenza A viruses in the background of A/PR/8/34 (PR8) (H1N1) and with NA genes obtained respectively from the 2009 pandemic H1N1 virus, a highly pathogenic avian H5N1 virus, and a lowly pathogenic avian H9N2 virus. These recombinant viruses, rPR8-H1N1NA, rPR8-H5N1NA, and rPR8-H9N2NA, were shown to have similar growth kinetics in cells and pathogenicity in mice. However, much more rPR8-H5N1NA and PR8-wt virions were released from chicken erythrocytes than virions of rPR8-H1N1NA and rPR8-H9N2NA after 1 h. In addition, in MDCK cells, rPR8-H5N1NA and rPR8-H9N2NA infected a higher percentage of cells, and induced cell-cell fusion faster and more extensively than PR8-wt and rPR8-H1N1NA did in the early phase of infection. In conclusion, NA replacement in this study did not affect virus replication kinetics but had different effects on infection initiation, virus release and fusion of infected cells. These phenomena might be partially due to NA proteins’ different specificity to α2-3/2-6-sialylated carbohydrate chains, but the exact mechanism remains to be explored.     

The Short Stalk Length of Highly Pathogenic Avian Influenza H5N1 Virus Neuraminidase Limits Transmission of Pandemic H1N1 Virus in Ferrets

H5N1 influenza viruses pose a pandemic threat but have not acquired the ability to support sustained transmission between mammals in nature. The restrictions to transmissibility of avian influenza viruses in mammals are multigenic, and overcoming them requires adaptations in hemagglutinin (HA) and PB2 genes. Here we propose that a further restriction to mammalian transmission of the majority of highly pathogenic avian influenza (HPAI) H5N1 viruses may be the short stalk length of the neuraminidase (NA) protein. This genetic feature is selected for when influenza viruses adapt to chickens. In our study, a recombinant virus with seven gene segments from a human isolate of the 2009 H1N1 pandemic combined with the NA gene from a typical chicken-adapted H5N1 virus with a short stalk did not support transmission by respiratory droplet between ferrets. This virus was also compromised in multicycle replication in cultures of human airway epithelial cells at 32°C. These defects correlated with a reduction in the ability of virus with a short-stalk NA to penetrate mucus and deaggregate virions. The deficiency in transmission and in cleavage of tethered substrates was overcome by increasing the stalk length of the NA protein. These observations suggest that H5N1 viruses that acquire a long-stalk NA through reassortment might be more likely to support transmission between humans. Phylogenetic analysis showed that reassortment with long-stalk NA occurred sporadically and as recently as 2011. However, all identified H5N1 viruses with a long-stalk NA lacked other mammalian adapting features and were thus several genetic steps away from becoming transmissible between humans.     

Growth determinants for H5N1 influenza vaccine seed viruses in MDCK cells

H5N1 influenza A viruses are exacting a growing human toll, with more than 240 fatal cases to date. In the event of an influenza pandemic caused by these viruses, embryonated chicken eggs, which are the approved substrate for human inactivated-vaccine production, will likely be in short supply because chickens will be killed by these viruses or culled to limit the worldwide spread of the infection. The Madin-Darby canine kidney (MDCK) cell line is a promising alternative candidate substrate because it supports efficient growth of influenza viruses compared to other cell lines. Here, we addressed the molecular determinants for growth of an H5N1 vaccine seed virus in MDCK cells, revealing the critical responsibility of the Tyr residue at position 360 of PB2, the considerable requirement for functional balance between hemagglutinin (HA) and neuraminidase (NA), and the partial responsibility of the Glu residue at position 55 of NS1. Based on these findings, we produced a PR8/H5N1 reassortant, optimized for this cell line, that derives all of its genes for its internal proteins from the PR8(UW) strain except for the NS gene, which derives from the PR8(Cambridge) strain; its N1 NA gene, which has a long stalk and derives from an early H5N1 strain; and its HA gene, which has an avirulent-type cleavage site sequence and is derived from a circulating H5N1 virus. Our findings demonstrate the importance and feasibility of a cell culture-based approach to producing seed viruses for inactivated H5N1 vaccines that grow robustly and in a timely, cost-efficient manner as an alternative to egg-based vaccine production.