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The incidence of human infection by zoonotic avian influenza viruses, especially H5N1 and H7N9 viruses, has increased. Current zoonotic H7N9 avian influenza viruses (identified since 2013) emerged during reassortment of viruses belonging to different subtypes. Despite analyses of their genetic background, we do not know why current H7N9 viruses are zoonotic. Therefore, there is a need to identify the factor(s) responsible for the extended host tropism that enables these viruses to infect humans as well as birds. To identify H7N9-specific amino acids that confer zoonotic properties on H7N9 viruses, we performed multiple alignment of the hemagglutinin (HA) amino acid sequences of A/Shanghai/1/2013 (H7N9) and A/duck/Zhejiang/12/2011(H7N3) (a putative, non- or less zoonotic HA donor to the zoonotic H7N9 virus). We also analyze the function of an H7N9 HA-specific amino acid with respect to HA acid stability, and evaluated the effect of acid stability on viral infectivity and virulence in a mouse model. HA2-116D, preserved in current zoonotic H7N9 viruses, was crucial for loss of HA acid stability. The acid-labile HA protein in H7 viruses played an important role in infection of human airway epithelial cells; HA2-116D contributed to infection and replication of H7 viruses. Finally, HA2-116D served as a H7 virulence factor in mice. These results suggest that acid-labile HA harboring HA2-116D confers zoonotic characteristics on H7N9 virus and that future novel zoonotic avian viruses could emerge from non-zoonotic H7 viruses via acquisition of mutations that remove HA acid stability.

人感染人畜共患禽流感病毒（zoonotic avian influenza viruses），尤其是H5N1与H7N9亚型病毒的病例数呈逐年上升趋势。2013年被首次鉴定的当前流行人畜共患H7N9禽流感病毒，系不同亚型病毒重配的产物。尽管已有针对其遗传背景的解析，但目前仍未阐明H7N9病毒具备人畜共患性的具体机制。因此，亟需明确赋予该病毒扩展宿主嗜性、可同时感染禽类与人类的关键决定因子。为筛选赋予H7N9病毒人畜共患特性的H7N9特异性氨基酸位点，本研究对A/上海/1/2013（H7N9）与A/鸭/浙江/12/2011（H7N3，该毒株为本次人畜共患H7N9病毒血凝素的潜在供体，本身几乎无或仅具备极弱的人畜共患性）的血凝素（hemagglutinin, HA）氨基酸序列开展了多重序列比对。本研究还针对某一H7N9病毒特异性血凝素氨基酸位点，探究了其对血凝素酸稳定性的影响，并在小鼠模型中评估了血凝素酸稳定性对病毒感染性与致病力的作用。存在于当前所有人畜共患H7N9病毒中的HA2-116D位点，是导致血凝素酸稳定性丧失的关键因素。H7亚型病毒中酸不稳定型血凝素蛋白，在感染人类气道上皮细胞的过程中发挥了关键作用；而HA2-116D位点可增强H7病毒的感染与复制能力。此外，在小鼠模型中，HA2-116D位点可作为H7亚型病毒的致病力因子。本研究结果表明，携带HA2-116D位点的酸不稳定型血凝素可赋予H7N9病毒人畜共患特性；同时提示，未来新型人畜共患禽流感病毒或可由非人畜共患的H7亚型病毒通过获得消除血凝素酸稳定性的突变而演化产生。