Structural Analysis of Human and Macaque mAbs 2909 and 2.5B: Implications for the Configuration of the Quaternary Neutralizing Epitope of HIV-1 gp120

The quaternary neutralizing epitope (ONE) of HIV-1 gp120 is preferentially expressed on the trimeric envelope spikes of intact HIV virions, and ONE-specific monoclonal antibodies (mAbs) potently neutralize HIV-1. Here, we present the crystal structures of the Fabs of human mAb 2909 and macaque mAb 2.5B. Both. mAbs have long beta hairpin CDR H3 regions >20 angstrom in length that are each situated at the center of their respective antigen-binding sites. Computational analysis showed that the paratopes include the whole CDR H3, while additional CDR residues form shallow binding pockets. Structural modeling suggests a way to understand the configuration of QNEs and the antigen-antibody interaction for ONE mAbs. Our data will be useful in designing immunogens that may elicit potent neutralizing QNE Abs.