Department of Virology, Research Institute for Microbial Diseases (BIKEN), Osaka University, Osaka, Japan.
PLoS Pathog. 2011 May;7(5):e1002068. doi: 10.1371/journal.ppat.1002068. Epub 2011 May 26.
Highly pathogenic avian influenza A virus subtype H5N1 is currently widespread in Asia, Europe, and Africa, with 60% mortality in humans. In particular, since 2009 Egypt has unexpectedly had the highest number of human cases of H5N1 virus infection, with more than 50% of the cases worldwide, but the basis for this high incidence has not been elucidated. A change in receptor binding affinity of the viral hemagglutinin (HA) from α2,3- to α2,6-linked sialic acid (SA) is thought to be necessary for H5N1 virus to become pandemic. In this study, we conducted a phylogenetic analysis of H5N1 viruses isolated between 2006 and 2009 in Egypt. The phylogenetic results showed that recent human isolates clustered disproportionally into several new H5 sublineages suggesting that their HAs have changed their receptor specificity. Using reverse genetics, we found that these H5 sublineages have acquired an enhanced binding affinity for α2,6 SA in combination with residual affinity for α2,3 SA, and identified the amino acid mutations that produced this new receptor specificity. Recombinant H5N1 viruses with a single mutation at HA residue 192 or a double mutation at HA residues 129 and 151 had increased attachment to and infectivity in the human lower respiratory tract but not in the larynx. These findings correlated with enhanced virulence of the mutant viruses in mice. Interestingly, these H5 viruses, with increased affinity to α2,6 SA, emerged during viral diversification in bird populations and subsequently spread to humans. Our findings suggested that emergence of new H5 sublineages with α2,6 SA specificity caused a subsequent increase in human H5N1 influenza virus infections in Egypt, and provided data for understanding the virus's pandemic potential.
高致病性禽流感病毒 H5N1 目前在亚洲、欧洲和非洲广泛传播，其在人类中的死亡率为 60%。特别是自 2009 年以来，埃及出人意料地成为全球感染 H5N1 病毒人数最多的国家，占全球病例的 50%以上，但导致高发病率的原因尚未阐明。病毒血凝素（HA）的受体结合亲和力从α2,3-到α2,6-连接的唾液酸（SA）的改变被认为是 H5N1 病毒成为大流行的必要条件。在这项研究中，我们对 2006 年至 2009 年在埃及分离的 H5N1 病毒进行了系统进化分析。系统进化分析结果表明，最近的人类分离株不成比例地聚集到几个新的 H5 亚系中，表明它们的 HAs 已经改变了受体特异性。通过反向遗传学，我们发现这些 H5 亚系获得了对α2,6 SA 的增强结合亲和力，同时保留了对α2,3 SA 的亲和力，并且确定了产生这种新受体特异性的氨基酸突变。HA 残基 192 发生单点突变或 HA 残基 129 和 151 发生双点突变的重组 H5N1 病毒对人类下呼吸道的附着和感染性增强，但对上呼吸道的附着和感染性没有增强。这些发现与突变病毒在小鼠中的毒力增强相关。有趣的是，这些与α2,6 SA 结合亲和力增强的 H5 病毒在禽类种群中的病毒多样化过程中出现，随后传播给人类。我们的研究结果表明，具有α2,6 SA 特异性的新 H5 亚系的出现导致埃及人类 H5N1 流感病毒感染的后续增加，并为了解病毒的大流行潜力提供了数据。
