Crystal structure and size-dependent neutralization properties of HK20, a human monoclonal antibody binding to the highly conserved heptad repeat 1 of gp41.

Charles Sabin,Gloria Agatic,Fabrizia Vanzetta,Federica Sallusto,Gabriella Scarlatti,Andreas Hinz,Robin Weiss,Antonio Lanzavecchia,David Lutje Hulsik,Chiara Silacci,Victor Buzon,Davide Corti,Mike S Seaman,Winfried Weissenhorn,Lara Mainetti

doi:10.1371/journal.ppat.1001195

Abstract

The human monoclonal antibody (mAb) HK20 neutralizes a broad spectrum of primary HIV-1 isolates by targeting the highly conserved heptad repeat 1 (HR1) of gp41, which is transiently exposed during HIV-1 entry. Here we present the crystal structure of the HK20 Fab in complex with a gp41 mimetic 5-Helix at 2.3 Å resolution. HK20 employs its heavy chain CDR H2 and H3 loops to bind into a conserved hydrophobic HR1 pocket that is occupied by HR2 residues in the gp41 post fusion conformation. Compared to the previously described HR1-specific mAb D5, HK20 approaches its epitope with a different angle which might favor epitope access and thus contribute to its higher neutralization breadth and potency. Comparison of the neutralization activities of HK20 IgG, Fab and scFv employing both single cycle and multiple cycle neutralization assays revealed much higher potencies for the smaller Fab and scFv over IgG, implying that the target site is difficult to access for complete antibodies. Nevertheless, two thirds of sera from HIV-1 infected individuals contain significant titers of HK20-inhibiting antibodies. The breadth of neutralization of primary isolates across all clades, the higher potencies for C-clade viruses and the targeting of a distinct site as compared to the fusion inhibitor T-20 demonstrate the potential of HK20 scFv as a therapeutic tool.

Highlights

The HIV-1 envelope (Env) glycoprotein is the main target for neutralizing antibodies
The HIV-1 envelope glycoprotein composed of the receptor binding subunit gp120 and the fusion protein gp41 is the prime target for neutralizing antibodies
Receptor binding induces a conformational change in gp41 that transiently exposes the conserved heptad repeat 1 (HR1) region

Summary

Introduction

The HIV-1 envelope (Env) glycoprotein is the main target for neutralizing antibodies. Together CD4 and co-receptor interactions are thought to induce conformational changes in the fusion protein subunit resulting in exposure and subsequent insertion of the fusion peptide into the target cell membrane which produce the fusion intermediate pre-hairpin structure that bridges viral and cellular membranes [10,11]. During this process heptad repeat regions 1 (HR1) and 2 (HR2) are transiently exposed [12] permitting interaction with peptide inhibitors of fusion such as T20 [13,14]. Subsequent refolding of the pre-hairpin structure into the post-fusion conformation [15,16,17,18] leads to the apposition of viral and cellular membranes catalyzing membrane fusion [19]

Methods

Results

Discussion

Conclusion