2013年以来，H7N9禽流感疫情多次暴发，其高致死率使得H7N9感染成为一个重要的公共卫生事件。流感病毒HA蛋白是流感病毒主要的免疫源，其抗原优势表位主要在HA的头部区，头部区高度可变性导致病毒很容易逃脱既往感染或疫苗免疫产生的免疫压力。尽管保守的HA的茎部能产生广谱性中和抗体，然而这类抗体在新的病毒感染时并没有起到有效的保护作用。因此阐明H7N9感染过程中B细胞响应机制对流感广谱性疫苗的设计具有重要指导意义。本研究拟利用流式分选结合单细胞测序技术从H7N9感染康复者B细胞中分离天然配对H7N9中和抗体，这些抗体能够真实反映人体B细胞对感染过程的响应；然后用结构生物学的手段鉴定抗原表位；再利用反向遗传技术构建不同表位抗体逃逸病毒株，研究人感染H7N9病毒后不同表位抗体产生的动力学过程，揭示B细胞的免疫应答机制。同时筛选到的中和抗体也为H7N9感染病人，尤其是重症病人提供治疗性候选药物。

Since 2013, a novel H7N9 avian influenza viruses have caused a major outbreak in humans in China. The high mortality of H7N9 infection has become a major public health event. The hemagglutinin (HA) of influenza virus is the major immunogen. Most strain-specific neutralizing antibodies formed in response to influenza vaccination or infection are directed toward highly variable and mutable regions of HA globular head region responsible for viral attachment to host cells. Influenza viruses reinfect previously immune hosts by acquiring escape mutations in the epitopes of neutralizing antibodies. The preexisting broadly neutralizing antibody targeting conserved stem region of HA can't prevent human from mutated viruses, but the mechanism is not clear. To clarify B cell response to H7N9 infection and guide the design of broadly influenza vaccines, H7N9 infected patient blood samples will be collected in this study. The neutralizing antibody level of serum and B cell response will be assessed. H7N9 neutralization antibodies will be isolated from H7N9 infected B cells by single cell sequencing Further, the structure of antigen and antibody complex will be solved. Based on the complex structure, antigenic determinant will be confirmed. Then we will generate a panel of escape virus strains which include key amino acid residue mutation of determinant using reverse genetics. These viruses in which one antigenic site is identical to the parental virus, whereas the other sites are antigenically altered will be used to analyze the dynamic process of neutralization antibodies in H7N9-infected patients and find immunodominant determinant of H7N9 avian influenza virus.