No abstract

Human antibody 2G12 neutralizes a broad range of human immunodeficiency virus type 1 (HIV-1) isolates by binding an unusually dense cluster of carbohydrate moieties on the "silent" face of the gp120 envelope glycoprotein. Crystal structures of Fab 2G12 and its complexes with the disaccharide Manalpha1-2Man and with the oligosaccharide Man9GlcNAc2 revealed that two Fabs assemble into an interlocked VH domain-swapped dimer. Further biochemical, biophysical, and mutagenesis data strongly support a Fab-dimerized antibody as the prevalent form that recognizes gp120. The extraordinary configuration of this antibody provides an extended surface, with newly described binding sites, for multivalent interaction with a conserved cluster of oligomannose type sugars on the surface of gp120. The unique interdigitation of Fab domains within an antibody uncovers a previously unappreciated mechanism for high-affinity recognition of carbohydrate or other repeating epitopes on cell or microbial surfaces.

The ability of human immunodeficiency virus (HIV-1) to persist and cause AIDS is dependent on its avoidance of antibody-mediated neutralization. The virus elicits abundant, envelope-directed antibodies that have little neutralization capacity. This lack of neutralization is paradoxical, given the functional conservation and exposure of receptor-binding sites on the gp120 envelope glycoprotein, which are larger than the typical antibody footprint and should therefore be accessible for antibody binding. Because gp120-receptor interactions involve conformational reorganization, we measured the entropies of binding for 20 gp120-reactive antibodies. Here we show that recognition by receptor-binding-site antibodies induces conformational change. Correlation with neutralization potency and analysis of receptor-antibody thermodynamic cycles suggested a receptor-binding-site 'conformational masking' mechanism of neutralization escape. To understand how such an escape mechanism would be compatible with virus-receptor interactions, we tested a soluble dodecameric receptor molecule and found that it neutralized primary HIV-1 isolates with great potency, showing that simultaneous binding of viral envelope glycoproteins by multiple receptors creates sufficient avidity to compensate for such masking. Because this solution is available for cell-surface receptors but not for most antibodies, conformational masking enables HIV-1 to maintain receptor binding and simultaneously to resist neutralization.

The identification and epitope mapping of broadly neutralizing anti-human immunodeficiency virus type 1 (HIV-1) antibodies (Abs) is important for vaccine design, but, despite much effort, very few such Abs have been forthcoming. Only one broadly neutralizing anti-gp41 monoclonal Ab (MAb), 2F5, has been described. Here we report on two MAbs that recognize a region immediately C-terminal of the 2F5 epitope. Both MAbs were generated from HIV-1-seropositive donors, one (Z13) from an antibody phage display library, and one (4E10) as a hybridoma. Both MAbs recognize a predominantly linear and relatively conserved epitope, compete with each other for binding to synthetic peptide derived from gp41, and bind to HIV-1 MN virions. By flow cytometry, these MAbs appear to bind relatively weakly to infected cells and this binding is not perturbed by pretreatment of the infected cells with soluble CD4. Despite the apparent linear nature of the epitopes of Z13 and 4E10, denaturation of recombinant envelope protein reduces the binding of these MAbs, suggesting some conformational requirements for full epitope expression. Most significantly, Z13 and 4E10 are able to neutralize selected primary isolates from diverse subtypes of HIV-1 (e.g., subtypes B, C, and E). The results suggest that a rather extensive region of gp41 close to the transmembrane domain is accessible to neutralizing Abs and could form a useful target for vaccine design.Eliciting broadly neutralizing antibodies (Abs) to human immunodeficiency virus (HIV-1) is a major goal of vaccine research but one that has proved extraordinarily elusive (8,11,77). This probably reflects the low antigenicity and immunogenicity of the HIV-1 envelope spike and most especially of relatively conserved regions of the spike. It is clear that much of the protein surface of the gp120 and gp41 protein molecules in the heterotrimeric envelope spike (gp120 3 -gp41 3 ) is directly or indirectly occluded from Ab binding by protein-protein interaction. Thus, for example, extensive surfaces on gp41 appear to be involved in interaction with other gp41 molecules and with gp120 (62, 63). Reciprocally, a portion of the surface of gp120 is occluded by the interaction with gp41 and by trimer formation (28,32,75,76). The relatively low immunogenicity of HIV-1 envelope trimers is also inferred from the low titers of neutralizing Abs, particularly cross-isolate neutralizing Abs, elicited during natural infection (31,39,40). This follows since a good correlation has been established between Ab neutralization and binding to envelope spikes, at least for T-cell-line-adapted viruses (50, 57, 61), suggesting that the deficit in neutralization originates from a deficit in spike binding. Low immunogenicity presumably arises, at least in part, from the weakly stimulating properties of the exposed regions of the envelope trimer. These include extensive regions of carbohydrate. A caveat here is that one cannot generally be sure of the eliciting antigen: Abs reactive with the trimer may have been elicited by other...