Complete functional mapping of infection- and vaccine-elicited antibodies against the fusion peptide of HIV

Eliciting broadly neutralizing antibodies (bnAbs) targeting envelope (Env) is a major goal of HIV vaccine development, but cross-clade breadth from immunization has only sporadically been observed. Recently, Xu et al (2018) elicited cross-reactive neutralizing antibody responses in a variety of animal models using immunogens based on the epitope of bnAb VRC34.01. The VRC34.01 antibody, which was elicited by natural human infection, targets the N terminus of the Env fusion peptide, a critical component of the virus entry machinery. Here we precisely characterize the functional epitopes of VRC34.01 and two vaccine-elicited murine antibodies by mapping all single amino-acid mutations to the BG505 Env that affect viral neutralization. While escape from VRC34.01 occurred via mutations in both fusion peptide and distal interacting sites of the Env trimer, escape from the vaccine-elicited antibodies was mediated predominantly by mutations in the fusion peptide. Cryo-electron microscopy of four vaccine-elicited antibodies in complex with Env trimer revealed focused recognition of the fusion peptide and provided a structural basis for development of neutralization breadth. Together, these functional and structural data suggest that the breadth of vaccine-elicited antibodies targeting the fusion peptide can be enhanced by specific interactions with additional portions of Env. Thus, our complete maps of viral escape both delineate pathways of resistance to these fusion peptide-directed antibodies and provide a strategy to improve the breadth or potency of future vaccine-induced antibodies against Env’s fusion peptide.诱导靶向包膜蛋白（envelope, Env）的广谱中和抗体（broadly neutralizing antibodies, bnAbs）是HIV疫苗研发的核心目标，但通过免疫接种诱导出的跨亚型中和广度仅偶有报道。近期，Xu等人（2018）采用基于广谱中和抗体VRC34.01表位的免疫原，在多种动物模型中成功诱导出了交叉反应性中和抗体应答。VRC34.01抗体由自然人类感染诱导产生，其靶向Env融合肽的N端——这是病毒入侵机器的关键组成部分。本研究通过定位BG505株Env上所有影响病毒中和作用的单氨基酸突变，精准表征了VRC34.01以及两种疫苗诱导鼠源抗体的功能性表位。研究发现，VRC34.01的逃逸可通过Env三聚体融合肽与远端相互作用位点的突变实现，而疫苗诱导抗体的逃逸则主要由融合肽内的突变介导。对四种与Env三聚体结合的疫苗诱导抗体开展冷冻电镜（cryo-electron microscopy）分析后发现，这些抗体聚焦识别融合肽，为中和广度的优化提供了结构依据。综上，这些功能性与结构性数据表明，通过与Env的额外区域形成特异性相互作用，可增强靶向融合肽的疫苗诱导抗体的中和广度。本研究完整的病毒逃逸图谱不仅阐明了针对这类融合肽靶向抗体的耐药通路，还为提升未来疫苗诱导的抗Env融合肽抗体的广度或效力提供了可行策略。