BackgroundThe isolation of human monoclonal antibodies (mAbs) that neutralize a broad spectrum of primary HIV-1 isolates and the characterization of the human neutralizing antibody B cell response to HIV-1 infection are important goals that are central to the design of an effective antibody-based vaccine.Methods and FindingsWe immortalized IgG+ memory B cells from individuals infected with diverse clades of HIV-1 and selected on the basis of plasma neutralization profiles that were cross-clade and relatively potent. Culture supernatants were screened using various recombinant forms of the envelope glycoproteins (Env) in multiple parallel assays. We isolated 58 mAbs that were mapped to different Env surfaces, most of which showed neutralizing activity. One mAb in particular (HJ16) specific for a novel epitope proximal to the CD4 binding site on gp120 selectively neutralized a multi-clade panel of Tier-2 HIV-1 pseudoviruses, and demonstrated reactivity that was comparable in breadth, but distinct in neutralization specificity, to that of the other CD4 binding site-specific neutralizing mAb b12. A second mAb (HGN194) bound a conserved epitope in the V3 crown and neutralized all Tier-1 and a proportion of Tier-2 pseudoviruses tested, irrespective of clade. A third mAb (HK20) with broad neutralizing activity, particularly as a Fab fragment, recognized a highly conserved epitope in the HR-1 region of gp41, but showed striking assay-dependent selectivity in its activity.ConclusionsThis study reveals that by using appropriate screening methods, a large proportion of memory B cells can be isolated that produce mAbs with HIV-1 neutralizing activity. Three of these mAbs show unusual breadth of neutralization and therefore add to the current panel of HIV-1 neutralizing antibodies with potential for passive protection and template-based vaccine design.

SUMMARY The restriction factor Bst2/tetherin contains two membrane anchors which are employed to retain some enveloped viruses including HIV-1 tethered to the plasma membrane in the absence of virus encoded antagonists. The 2.77 Å crystal structure of the extracellular core presented here reveals a parallel 90 Å long disulfide linked coiled-coil domain while the complete extracellular domain forms an extended 170 Å long rod-like structure based on small angle X-ray scattering data. Mutagenesis analyses indicate that both the coiled-coil and the N-terminal region are required for retention of HIV-1, suggesting that the elongated structure can function as a molecular ruler to bridge long distances. The structure reveals substantial irregularities and instabilities throughout the coiled-coil, which contribute to its low stability in the absence of disulfide bonds. We propose that the irregular coiled-coil provides conformational flexibility and ensures that Bst2/tetherin anchoring in the plasma and the newly formed virus membrane do not interfere with budding.