Structural Basis for Enhanced HIV-1 Neutralization by a Dimeric Immunoglobulin G Form of the Glycan-Recognizing Antibody 2G12

Creators: Wu, Yunji; West, Anthony P., Jr.; Kim, Helen J.; Thornton, Matthew E.; Ward, Andrew B.; Bjorkman, Pamela J.

Abstract

The human immunoglobulin G (IgG) 2G12 recognizes high-mannose carbohydrates on the HIV type 1 (HIV-1) envelope glycoprotein gp120. Its two antigen-binding fragments (Fabs) are intramolecularly domain exchanged, resulting in a rigid (Fab)_2 unit including a third antigen-binding interface not found in antibodies with flexible Fab arms. We determined crystal structures of dimeric 2G12 IgG created by intermolecular domain exchange, which exhibits increased breadth and >50-fold increased neutralization potency compared with monomeric 2G12. The four Fab and two fragment crystalline (Fc) regions of dimeric 2G12 were localized at low resolution in two independent structures, revealing IgG dimers with two (Fab)_2 arms analogous to the Fabs of conventional monomeric IgGs. Structures revealed three conformationally distinct dimers, demonstrating flexibility of the (Fab)_2-Fc connections that was confirmed by electron microscopy, small-angle X-ray scattering, and binding studies. We conclude that intermolecular domain exchange, flexibility, and bivalent binding to allow avidity effects are responsible for the increased potency and breadth of dimeric 2G12.

Additional Information

© 2013 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. Received: August 20, 2013. Revised: October 25, 2013. Accepted: November 6, 2013. Published: December 5, 2013. We thank Jost Vielmetter and the Caltech Protein Expression Center for assistance with Biacore studies and protein expression; Jens Kaiser, Pavle Nikolovski, the Caltech Molecular Observatory, the 2012 Advanced Photon Source (APS) Data Collection Workshop, and the CCP4 School at Argonne National Laboratory for help with crystallographic studies; the staff at Stanford Synchrotron Radiation Lightsource (SSRL) Beamlines 4-2 and 12-2 for assistance with SAXS studies; the staff at Beamlines 12-2 (SSRL) and APS General Medical Sciences and Cancer Institutes Structural Biology Facility Beamlines 231D-B and 231D-D for assistance with SAXS and crystallographic data collection; Justin Chartron for help with SAXS and crystallographic data analysis; the developers of PHENIX, CCP4, and XDS software for assistance and advice concerning X-ray crystallography; Leo Stamatatos, Zara Fulton, Reza Khayat, Gloria Tran, and members of the Bjorkman laboratory for protein reagents; William Lange, Marta Murphy, and Maria Politzer for help making figures and models; and Andrew Davenport and Beth Stadtmueller for critical reading of the manuscript. This work was supported by a Collaboration for AIDS Vaccine Discovery grant from The Bill and Melinda Gates Foundation (grant ID 1040753 to P.J.B.), the National Institutes of Health (2 R37AI041239-06A1 to P.J.B.), and startup funds from the Scripps Research Institute (to A.B.W.).

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