Department of Biochemistry and Molecular Pharmacology, NYU School of Medicine, New York, New York, USA.
Department of Biomedical Sciences, Iowa State University, Ames, Iowa, USA.
J Virol. 2018 Mar 14;92(7). doi: 10.1128/JVI.01894-17. Print 2018 Apr 1.
The third variable (V3) loop of HIV-1 gp120 is an immunodominant region targeted by neutralizing antibodies (nAbs). Despite limited breadth, better characterization of the structural details of the interactions between these nAbs and their target epitopes would enhance our understanding of the mechanism of neutralization and facilitate designing better immunogens to induce nAbs with greater breadth. Recently, we isolated two anti-V3 neutralizing monoclonal antibodies (MAbs), 10A3 and 10A37, from a rabbit immunized with gp120 of the M group consensus sequence. In this study, crystal structures of these MAbs bound to target epitopes were determined. 10A3 binds to the V3 crown (TRKSIHIGPGRAF) using the cradle binding mode, similar to human V3 MAbs encoded by IGHV5-51 germ line genes, and its epitope structure resembles that bound to the human antibodies. In contrast, 10A37, which exhibits greater breadth and potency than 10A3, binds the V3 crown and the succeeding stem region (HIGPGRAFYTTGEI). Unexpectedly, the RAFYTT portion of the epitope existed as helical turns, a V3 structure that has not been observed previously. Its main chain-dominated antigen-antibody interactions not only explain the broad neutralization of 10A37 but also show that its epitope is a potential vaccine target to be further evaluated. In conclusion, our study provides novel insights about neutralization-susceptible epitope structures of the V3 loop of HIV-1 gp120 and demonstrates that, despite low amino acid sequence similarity to human antibody germ line genes, rabbits can serve as a useful animal model to evaluate human vaccine candidates. The apex crown of V3 of HIV-1 gp120 is the most immunogenic region of the surface glycoprotein, and many MAbs targeting this region have been developed. Structural understanding of V3 crown MAbs not only can help understand how antibody responses target this unique region but also contribute to immunogen design for vaccine development. We present here crystal structures of two neutralizing V3 MAbs, 10A3 and 10A37, developed from a rabbit immunized with gp120. Our analysis of 10A3 in complex with V3 provided a detailed example of how epitope complexity can evolve with affinity maturation, while that of 10A37 revealed a novel V3 binding mode targeting the C-terminal side of the V3 crown and showed that this region can form a helical structure. Our study provides novel insights about neutralization-susceptible V3 epitope structures and demonstrates that rabbits can serve as a useful animal model to evaluate human vaccine candidates.
HIV-1 gp120 的第三可变环（V3）是中和抗体（nAb）靶向的免疫优势区域。尽管中和抗体的广谱性有限，但更好地描述这些 nAb 与其靶表位之间的结构细节，将有助于我们理解中和机制，并促进设计具有更大广谱性的更好免疫原。最近，我们从用 M 组共识序列 gp120 免疫的兔子中分离出两种抗 V3 中和单克隆抗体（mAb），10A3 和 10A37。在这项研究中，我们测定了这些 mAb 与靶表位结合的晶体结构。10A3 采用摇篮结合模式与 V3 冠（TRKSIHIGPGRAF）结合，类似于由 IGHV5-51 胚系基因编码的人类 V3 mAb，其表位结构类似于与人抗体结合的结构。相比之下，10A37 比 10A3 具有更大的广谱性和效力，它与 V3 冠和随后的茎区（HIGPGRAFYTTGEI）结合。出乎意料的是，抗原表位的 RAFYTT 部分存在为螺旋转角，这是以前未观察到的 V3 结构。其主要链主导的抗原-抗体相互作用不仅解释了 10A37 的广谱中和作用，还表明其表位是一个潜在的疫苗靶标，值得进一步评估。总之，我们的研究提供了关于 HIV-1 gp120 V3 环中和敏感表位结构的新见解，并表明，尽管与人类抗体胚系基因的氨基酸序列相似性较低，但兔子可以作为评估人类疫苗候选物的有用动物模型。HIV-1 gp120 的 V3 顶端冠是表面糖蛋白最具免疫原性的区域，已经开发出许多针对该区域的 mAb。对 V3 冠 mAb 的结构理解不仅有助于了解抗体反应如何靶向这一独特区域，还有助于疫苗开发的免疫原设计。我们在这里介绍了两种中和 V3 mAb，10A3 和 10A37 的晶体结构，它们是由用 gp120 免疫的兔子产生的。我们对与 V3 复合的 10A3 的分析提供了一个详细的例子，说明了表位复杂性如何随着亲和力成熟而进化，而对 10A37 的分析则揭示了一种针对 V3 冠 C 末端的新型 V3 结合模式，并表明该区域可以形成螺旋结构。我们的研究提供了关于中和敏感 V3 表位结构的新见解，并表明兔子可以作为评估人类疫苗候选物的有用动物模型。
