The trimeric envelope (Env) spike is the focus of
vaccine design efforts aimed at generating
broadly neutralizing antibodies (
bNAbs) to protect against HIV-1
infection. Three recent developments have facilitated a thorough investigation of the antigenic structure of the Env trimer: 1) the isolation of many
bNAbs against multiple different
epitopes; 2) the generation of a soluble trimer mimic, BG505 SOSIP.664
gp140, that expresses most
bNAb epitopes; 3) facile binding assays involving the oriented immobilization of tagged trimers. Using these tools, we generated an antigenic map of the trimer by antibody cross-competition. Our analysis delineates three well-defined
epitope clusters (CD4 binding site, quaternary V1V2 and Asn332-centered oligomannose patch) and new
epitopes at the gp120-gp41 interface. It also identifies the relationships among these clusters. In addition to
epitope overlap, we defined three more ways in which
antibodies can cross-compete: steric competition from binding to proximal but non-overlapping
epitopes (e.g., PGT151 inhibition of 8ANC195 binding); allosteric inhibition (e.g., PGT145 inhibition of 1NC9, 8ANC195, PGT151 and CD4 binding); and competition by reorientation of
glycans (e.g., PGT135 inhibition of CD4bs
bNAbs, and CD4bs
bNAb inhibition of 8ANC195). We further demonstrate that
bNAb binding can be complex, often affecting several other areas of the trimer surface beyond the
epitope. This extensive analysis of the antigenic structure and the
epitope interrelationships of the Env trimer should aid in design of both
bNAb-based
therapies and
vaccines intended to induce
bNAbs.