Institute of Human Virology, Department of Medicine, University of Maryland School of Medicine, University of Maryland, Baltimore, 725 West Lombard Street, Baltimore, MD 21201, USA.
J Virol. 2011 Dec;85(24):13097-104. doi: 10.1128/JVI.05072-11. Epub 2011 Oct 12.
Certain antibodies from HIV-infected humans bind conserved transition state (CD4 induced [CD4i]) domains on the HIV envelope glycoprotein, gp120, and demonstrate extreme dependence on the formation of a gp120-human CD4 receptor complex. The epitopes recognized by these antibodies remain undefined although recent crystallographic studies of the anti-CD4i monoclonal antibody (MAb) 21c suggest that contacts with CD4 as well as gp120 might occur. Here, we explore the possibility of hybrid epitopes that demand the collaboration of both gp120 and CD4 residues to enable antibody reactivity. Analyses with a panel of human anti-CD4i MAbs and gp120-CD4 antigens with specific mutations in predicted binding domains revealed one putative hybrid epitope, defined by the human anti-CD4i MAb 19e. In virological and immunological tests, MAb 19e did not bind native or constrained gp120 except in the presence of CD4. This contrasted with other anti-CD4i MAbs, including MAb 21c, which bound unliganded, full-length gp120 held in a constrained conformation. Conversely, MAb 19e exhibited no specific reactivity with free human CD4. Computational modeling of MAb 19e interactions with gp120-CD4 complexes suggested a distinct binding profile involving antibody heavy chain interactions with CD4 and light chain interactions with gp120. In accordance, targeted mutations in CD4 based on this model specifically reduced MAb 19e interactions with stable gp120-CD4 complexes that retained reactivity with other anti-CD4i MAbs. These data represent a rare instance of an antibody response that is specific to a pathogen-host cell protein interaction and underscore the diversity of immunogenic CD4i epitope structures that exist during natural infection.
某些来自 HIV 感染者的抗体结合 HIV 包膜糖蛋白 gp120 上保守的过渡态（CD4 诱导 [CD4i]）结构域，并表现出对 gp120-人 CD4 受体复合物形成的极端依赖性。尽管最近对抗 CD4i 单克隆抗体（MAb）21c 的晶体学研究表明，可能与 CD4 以及 gp120 发生接触，但这些抗体识别的表位仍未确定。在这里，我们探讨了需要 gp120 和 CD4 残基共同协作才能产生抗体反应的杂交表位的可能性。用一组人抗 CD4i MAb 和 gp120-CD4 抗原进行分析，这些抗原在预测的结合结构域中具有特定的突变，揭示了一个可能的杂交表位，由人抗 CD4i MAb 19e 定义。在病毒学和免疫学测试中，除非存在 CD4，否则 MAb 19e 不会结合天然或约束的 gp120。这与其他抗 CD4i MAb 形成对比，包括 MAb 21c，它结合无配体、保持在约束构象的全长 gp120。相反，MAb 19e 与游离的人 CD4 没有特异性反应。对 MAb 19e 与 gp120-CD4 复合物相互作用的计算建模表明，存在一种独特的结合模式，涉及抗体重链与 CD4 的相互作用以及轻链与 gp120 的相互作用。相应地，基于该模型的针对 CD4 的靶向突变特异性降低了 MAb 19e 与保留与其他抗 CD4i MAb 反应性的稳定 gp120-CD4 复合物的相互作用。这些数据代表了一种罕见的抗体反应，该反应特异性针对病原体-宿主细胞蛋白相互作用，并强调了在自然感染期间存在的多样化的免疫原性 CD4i 表位结构。
