Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
mBio. 2023 Feb 28;14(1):e0337022. doi: 10.1128/mbio.03370-22. Epub 2023 Jan 11.
HIV-1 and its SIV precursors share a broadly neutralizing antibody (bNAb) epitope in variable loop 2 (V2) at the envelope glycoprotein (Env) trimer apex. Here, we tested the immunogenicity of germ line-targeting versions of a chimpanzee SIV (SIVcpz) Env in human V2-apex bNAb heavy-chain precursor-expressing knock-in mice and as chimeric simian-chimpanzee immunodeficiency viruses (SCIVs) in rhesus macaques (RMs). Trimer immunization of knock-in mice induced V2-directed NAbs, indicating activation of V2-apex bNAb precursor-expressing mouse B cells. SCIV infection of RMs elicited high-titer viremia, potent autologous tier 2 neutralizing antibodies, and rapid sequence escape in the canonical V2-apex epitope. Six of seven animals also developed low-titer heterologous plasma breadth that mapped to the V2-apex. Antibody cloning from two of these animals identified multiple expanded lineages with long heavy chain third complementarity determining regions that cross-neutralized as many as 7 of 19 primary HIV-1 strains, but with low potency. Negative stain electron microscopy (NSEM) of members of the two most cross-reactive lineages confirmed V2 targeting but identified an angle of approach distinct from prototypical V2-apex bNAbs, with antibody binding either requiring or inducing an occluded-open trimer. Probing with conformation-sensitive, nonneutralizing antibodies revealed that SCIV-expressed, but not wild-type SIVcpz Envs, as well as a subset of primary HIV-1 Envs, preferentially adopted a more open trimeric state. These results reveal the existence of a cryptic V2 epitope that is exposed in occluded-open SIVcpz and HIV-1 Env trimers and elicits cross-neutralizing responses of limited breadth and potency. An effective HIV-1 vaccination strategy will need to stimulate rare precursor B cells of multiple bNAb lineages and affinity mature them along desired pathways. Here, we searched for V2-apex germ line-targeting Envs among a large set of diverse primate lentiviruses and identified minimally modified versions of one chimpanzee SIV Env that bound several human V2-apex bNAb precursors and stimulated one of these in a V2-apex bNAb precursor-expressing knock-in mouse. We also generated chimeric simian-chimpanzee immunodeficiency viruses and showed that they elicit low-titer V2-directed heterologous plasma breadth in six of seven infected rhesus macaques. Characterization of this antibody response identified a new class of weakly cross-reactive neutralizing antibodies that target the V2-apex, but only in occluded-open Env trimers. The existence of this cryptic epitope, which in some Env backgrounds is immunodominant, needs to be considered in immunogen design.
HIV-1 及其 SIV 前体在包膜糖蛋白 (Env) 三聚体顶端的可变环 2 (V2) 中共享一个广泛中和抗体 (bNAb) 表位。在这里，我们测试了在人类 V2-顶点 bNAb 重链前体表达敲入小鼠中靶向种系的黑猩猩 SIV (SIVcpz) Env 的免疫原性，以及在恒河猴 (RMs) 中作为嵌合性猿猴-黑猩猩免疫缺陷病毒 (SCIV) 的免疫原性。敲入小鼠的三聚体免疫诱导了针对 V2 的 NAb，表明激活了 V2-顶点 bNAb 前体表达的小鼠 B 细胞。SCIV 感染 RM 引发了高滴度的病毒血症、强效的自体 tier 2 中和抗体和在规范的 V2-顶点表位中的快速序列逃逸。七只动物中有六只还产生了针对 V2-顶点的低滴度异种血浆广度。从其中两只动物的抗体克隆中鉴定出多个具有长重链第三互补决定区的扩增谱系，这些谱系可以中和多达 19 种原发性 HIV-1 株中的 7 种，但效力较低。两种最具交叉反应性谱系成员的负染电子显微镜 (NSEM) 确认了 V2 的靶向性，但确定了一个与典型的 V2-顶点 bNAb 不同的接近角度，抗体结合要么需要要么诱导封闭-开放的三聚体。用构象敏感、非中和抗体探测发现，SCIV 表达的但不是野生型 SIVcpz Env，以及一部分原发性 HIV-1 Env，优先采用更开放的三聚体状态。这些结果揭示了存在一个隐藏的 V2 表位，该表位在封闭-开放的 SIVcpz 和 HIV-1 Env 三聚体中暴露，并引发了有限广度和效力的交叉中和反应。有效的 HIV-1 疫苗接种策略将需要刺激多种 bNAb 谱系的稀有前体 B 细胞，并沿着所需的途径使它们成熟。在这里，我们在一大组不同的灵长类慢病毒中搜索 V2-顶点种系靶向 Env，并鉴定出一种经过最小修饰的黑猩猩 SIV Env，它可以结合几种人类 V2-顶点 bNAb 前体，并在 V2-顶点 bNAb 前体表达敲入小鼠中刺激其中一种。我们还生成了嵌合性猿猴-黑猩猩免疫缺陷病毒，并表明它们在感染的七只恒河猴中引发了低滴度的针对 V2 的异种血浆广度。对这种抗体反应的特征分析确定了一类新的弱交叉反应性中和抗体，这些抗体针对 V2-顶点，但仅在封闭-开放的 Env 三聚体中。在免疫原设计中需要考虑到这个隐藏表位的存在，在某些 Env 背景下，它是免疫显性的。
