Understanding how broadly neutralizing antibodies (bnAbs) to influenza hemagglutinin (HA) naturally develop in humans is critical to the design of universal influenza vaccines. Several classes of bnAbs directed to the conserved HA stem were found in multiple individuals, including one encoded by heavy-chain variable domain V_H6-1. We describe two genetically similar V_H6-1 bnAb clonotypes from the same individual that exhibit different developmental paths toward broad neutralization activity. One clonotype evolved from a germline precursor recognizing influenza group 1 subtypes to gain breadth to group 2 subtypes. The other clonotype recognized group 2 subtypes and developed binding to group 1 subtypes through somatic hypermutation. Crystal structures reveal that the specificity differences are primarily mediated by complementarity-determining region H3 (CDR H3). Thus, while V_H6-1 provides a framework for development of HA stem-directed bnAbs, sequence differences in CDR H3 junctional regions during VDJ recombination can alter reactivity and evolutionary pathways toward increased breadth.

了解通用的人类流感血凝素（HA）中和抗体（bnAbs）的广泛发育对于通用流感疫苗的设计至关重要。在多个个体中发现了几类针对保守的HA茎的bnAb，其中包括由重链可变域V _H 6-1编码的bnAb 。我们描述两个遗传相似的V _H6-1来自同一个体的bnAb克隆型表现出朝向广泛的中和活性的不同发育途径。一种克隆型从识别流感第1组亚型的种系前体演变为获得第2组亚型的广度。其他克隆型识别第2组亚型，并通过体细胞超突变形成与第1组亚型的结合。晶体结构表明，特异性差异主要由互补决定区H3（CDR H3）介导。因此，虽然V _H 6-1为HA干定向bnAb的发展提供了框架，但在VDJ重组过程中CDR H3连接区的序列差异可以改变反应性和向广度发展的进化途径。