State Key Laboratory of Veterinary Biotechnology, Harbin Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Harbin, China.
State Key Laboratory of Veterinary Biotechnology, Harbin Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Harbin, China
J Virol. 2020 Apr 16;94(9). doi: 10.1128/JVI.01836-19.
Porcine circovirus type 2 (PCV2) is an important pathogen in swine herds, and its infection of pigs has caused severe economic losses to the pig industry worldwide. The capsid protein of PCV2 is the only structural protein that is associated with PCV2 infection and immunity. Here, we report a neutralizing monoclonal antibody (MAb), MAb 3A5, that binds to intact PCV2 virions of the PCV2a, PCV2b, and PCV2d genotypes. MAb 3A5 neutralized PCV2 by blocking viral attachment to PK15 cells. To further explore the neutralization mechanism, we resolved the structure of the PCV2 virion in complex with MAb 3A5 Fab fragments by using cryo-electron microscopy single-particle analysis. The binding sites were located at the topmost edges around 5-fold icosahedral symmetry axes, with each footprint covering amino acids from two adjacent capsid proteins. Most of the epitope residues (15/18 residues) were conserved among 2,273 PCV2 strains. Mutations of some amino acids within the epitope had significant effects on the neutralizing activity of MAb 3A5. This study reveals the molecular and structural bases of this PCV2-neutralizing antibody and provides new and important information for vaccine design and therapeutic antibody development against PCV2 infections. PCV2 is associated with several clinical manifestations collectively known as PCV2-associated diseases (PCVADs). Neutralizing antibodies play a crucial role in the prevention of PCVADs. We demonstrated previously that a MAb, MAb 3A5, neutralizes the PCV2a, PCV2b, and PCV2d genotypes with different degrees of efficiency, but the underlying mechanism remains elusive. Here, we report the neutralization mechanism of this MAb and the structure of the PCV2 virion in complex with MAb 3A5 Fabs, showing a binding mode in which one Fab interacted with more than two loops from two adjacent capsid proteins. This binding mode has not been observed previously for PCV2-neutralizing antibodies. Our work provides new and important information for vaccine design and therapeutic antibody development against PCV2 infections.
猪圆环病毒 2 型（PCV2）是猪群中的一种重要病原体，其感染猪只给全球养猪业造成了严重的经济损失。PCV2 的衣壳蛋白是唯一与 PCV2 感染和免疫相关的结构蛋白。在这里，我们报告了一种针对 PCV2a、PCV2b 和 PCV2d 基因型完整 PCV2 病毒粒子的中和性单克隆抗体（MAb），MAb 3A5。MAb 3A5 通过阻断病毒与 PK15 细胞的附着来中和 PCV2。为了进一步探索中和机制，我们使用冷冻电镜单颗粒分析解析了与 MAb 3A5 Fab 片段复合物的 PCV2 病毒粒子的结构。结合部位位于 5 重旋转对称轴的最高边缘周围，每个足迹覆盖两个相邻衣壳蛋白的氨基酸。表位中的大多数氨基酸残基（15/18 个残基）在 2273 株 PCV2 株中保守。表位内一些氨基酸的突变对 MAb 3A5 的中和活性有显著影响。这项研究揭示了这种 PCV2 中和抗体的分子和结构基础，并为 PCV2 感染的疫苗设计和治疗性抗体开发提供了新的重要信息。PCV2 与几种临床表现有关，统称为 PCV2 相关疾病（PCVADs）。中和抗体在预防 PCVADs 中发挥着至关重要的作用。我们之前已经证明，一种 MAb，MAb 3A5，以不同的效率中和 PCV2a、PCV2b 和 PCV2d 基因型，但潜在的机制仍不清楚。在这里，我们报告了这种 MAb 的中和机制和与 MAb 3A5 Fabs 复合物的 PCV2 病毒粒子的结构，显示了一种结合模式，其中一个 Fab 与来自两个相邻衣壳蛋白的两个以上环相互作用。这种结合模式以前在 PCV2 中和抗体中没有观察到。我们的工作为 PCV2 感染的疫苗设计和治疗性抗体开发提供了新的重要信息。
