We recently have characterized a comprehensive panel of human mAbs isolated from HIV-1 infected individuals based on a modified antibody-dependent cell-mediated cytotoxicity (ADCC) assay that detects epitopes exposed on the target cell surface during viral entry. Among the epitopes that constituted the best ADCC targets were those exposed on trimeric Env only after CD4 engagement and association with the N- and C- termini of gp120. Here we present studies, which elucidate for the first time, the structural signatures of these novel transitional ADCC target epitopes. Crystal structures of ternary complexes formed between antibodies against these epitopes, monomeric gp120 and the host receptor CD4 or a CD4 peptide mimetic have been determined to provide atomic resolution insight into the recognition sites. The complex structures reveal that the contact surfaces on gp120 of each of mAb tested map exclusively to the gp120 inner domain and include elements of the gp41-interactive region, proximal to the N-terminus of the mature protein. Analysis of these structures in the context of cryo-EM structures of virion-associated HIV envelope trimers indicated that these partially overlapping epitope surfaces are occluded from antibodies by gp41 in the trimeric Env even in the presence of soluble CD4. They become exposed only upon binding to cell surface CD4, as we confirmed by flow cytometry with surface-expressed trimers and by confocal microscopy of attached virions. Collectively, these data show that in the context of a viral trimer the exposure of certain gp120 epitopes selectively depends upon binding to cell surface CD4. The fact that these epitopes are targets for antiviral antibodies has important implications for understanding humoral anti-HIV immunity and for HIV vaccine design.
© 2013 Lippincott Williams & Wilkins, Inc.