Book excerpt: Although the ability to elicit broadly neutralizing antibodies is considered an important feature of an effective HIV-1 vaccine, elicitation of such antibodies by vaccine antigens remains elusive. To better understand the regions of the HIV-1 envelope spike that are vulnerable to neutralizing antibodies, and the requirements for broad neutralization, the antibody repertoires of three asymptomatic, HIV-1 infected individuals with broadly neutralizing sera were characterized. We demonstrate that neutralization arises from the Protein-A reactive IgG fraction of the sera. Neutralization of a panel of clade B viruses was not inhibited by linear peptides corresponding to the third hypervariable (V3) loop of gp120, suggesting that V3-specific antibodies do not mediate cross-neutralization. The sera also failed to significantly neutralize chimeric SIV and HIV-2 viruses displaying conserved HIV-1 gp41 neutralizing epitopes, indicating that antibodies directed towards these epitopes also do not account for the broad neutralization. Fractionation of polyclonal IgG on recombinant monomeric clade B gp120 demonstrates that gp120-reactive antibodies mediate neutralization of some isolates, but not all. Taken together, the data suggest that neutralization results from multiple neutralizing specificities, although the total number of different specificities is likely small. The neutralization breadth most likely arises from recognition of the CD4 binding site of gp120, although carbohydrate epitopes and conformational epitopes specific to trimeric envelope cannot be excluded. To isolate the monoclonal specificities responsible for neutralization of HIV-1[subscript]JR-FL, phage-displayed antibody libraries representing the repertoire of one donor were selected against clade B gp120 proteins used in gp120-fractionation experiments. However, only non-neutralizing antibodies specific for the CD4 binding site and V3-loop were recovered. Rescue of the broadly neutralizing monoclonal antibodies may require the development of novel screening antigens and the use of eukaryotic antibody rescue technologies. Lastly, the structure of the V3-specific antibody 447-52D, which exhibits a neutralization breadth uncharacteristic of V3-antibodies, in complex with a V3-peptide is reported. The structure confirms that the neutralization breadth is due to the use of main-chain hydrogen bonds in antigen binding. The similarity of this structure to previously reported V3-peptide structures suggests that structural features of the V3-loop crown are highly conserved. Thus, although V3-specific antibodies do not mediate broad serum neutralization, the V3-region might potentially provide some degree of protection in a multi-component vaccine.