B Cell Recognition of the Conserved HIV-1 Co-Receptor Binding Site Is Altered by Endogenous Primate CD4

The surface HIV-1 exterior envelope glycoprotein, gp120, binds to CD4 on the target cell surface to induce the co-receptor binding site on gp120 as the initial step in the entry process. The binding site is comprised of a highly conserved region on the gp120 core, as well as elements of the third variable region (V3). Antibodies against the co-receptor binding site are abundantly elicited during natural infection of humans, but the mechanism of elicitation has remained undefined. In this study, we investigate the requirements for elicitation of co-receptor binding site antibodies by inoculating rabbits, monkeys and human-CD4 transgenic (huCD4) rabbits with envelope glycoprotein (Env) trimers possessing high affinity for primate CD4. A cross-species comparison of the antibody responses showed that similar HIV-1 neutralization breadth was elicited by Env trimers in monkeys relative to wild-type (WT) rabbits. In contrast, antibodies against the co-receptor site on gp120 were elicited only in monkeys and huCD4 rabbits, but not in the WT rabbits. This was supported by the detection of high-titer co-receptor antibodies in all sera from a set derived from human volunteers inoculated with recombinant gp120. These findings strongly suggest that complexes between Env and (high-affinity) primate CD4 formed in vivo are responsible for the elicitation of the co-receptor-site-directed antibodies. They also imply that the naïve B cell receptor repertoire does not recognize the gp120 co-receptor site in the absence of CD4 and illustrate that conformational stabilization, imparted by primary receptor interaction, can alter the immunogenicity of a type 1 viral membrane protein.

作为HIV-1入侵宿主细胞的初始步骤，HIV-1包膜外表面糖蛋白gp120（surface HIV-1 exterior envelope glycoprotein）会与靶细胞表面的CD4受体结合，从而诱导gp120上形成共受体结合位点。该结合位点由gp120核心区域的高度保守区段，以及第三可变区（V3）的结构元件共同构成。在人类自然感染HIV-1的过程中，机体会大量诱导产生针对该共受体结合位点的抗体，但其具体诱导机制至今仍不明确。本研究中，我们通过向家兔、猕猴以及人CD4转基因（huCD4）家兔接种对灵长类CD4具有高亲和力的包膜糖蛋白（Env）三聚体，探究了诱导共受体结合位点抗体产生的必要条件。对不同物种的抗体应答进行跨物种比较后发现，与野生型（WT）家兔相比，Env三聚体在猕猴体内可诱导出相当的HIV-1中和广度。与之相反，针对gp120上共受体位点的抗体仅在猕猴和huCD4家兔体内被诱导产生，野生型家兔则未出现此类抗体应答。在一组接种重组gp120的人类志愿者血清样本中，所有血清均检测到高滴度的共受体抗体，这一结果进一步佐证了上述结论。本研究结果强烈提示，体内形成的Env与（高亲和力）灵长类CD4复合物是诱导共受体位点靶向性抗体产生的核心原因。该研究同时表明，在缺乏CD4的情况下，初始B细胞受体库无法识别gp120的共受体位点；此外还证实，初级受体结合所赋予的构象稳定作用，可改变1型病毒膜蛋白的免疫原性。