Structural analysis of a novel rabbit monoclonal antibody R53 targeting an epitope in HIV-1 gp120 C4 region critical for receptor and co-receptor binding

Ruimin Pan,Yuxin Chen,Michael Vaine,Guangnan Hu,Shixia Wang,Shan Lu,Xiang-Peng Kong

doi:10.1038/emi.2015.44

Abstract

The fourth conserved region (C4) in the HIV-1 envelope glycoprotein (Env) gp120 is a structural element that is important for its function, as it binds to both the receptor CD4 and the co-receptor CCR5/CXCR4. It has long been known that this region is highly immunogenic and that it harbors B-cell as well as T-cell epitopes. It is the target of a number of antibodies in animal studies, which are called CD4-blockers. However, the mechanism by which the virus shields itself from such antibody responses is not known. Here, we determined the crystal structure of R53 in complex with its epitope peptide using a novel anti-C4 rabbit monoclonal antibody R53. Our data show that although the epitope of R53 covers a highly conserved sequence 433AMYAPPI439, it is not available in the gp120 trimer and in the CD4-bound conformation. Our results suggest a masking mechanism to explain how HIV-1 protects this critical region from the human immune system.

Highlights

The HIV-1 envelope glycoprotein (Env) gp[120] initiates viral entry into host cells by binding to its receptor CD4 and to its co-receptor CCR5/CXCR4, and it is the major target for acquired immune deficiency syndrome vaccine development
Our results suggest a masking mechanism to explain how HIV-1 protects this critical region from the human immune system
The CD4 receptor-binding site is protected by entropy masking,[1] and the co-receptor-binding site in the pre-fusion complex is completely buried under variable loops.[6,7,8]

Summary

Introduction

The HIV-1 envelope glycoprotein (Env) gp[120] initiates viral entry into host cells by binding to its receptor CD4 and to its co-receptor CCR5/CXCR4, and it is the major target for acquired immune deficiency syndrome vaccine development. Conformational masking, by either covering immunogenic epitope regions with other domains, or by having them adopt different conformations, is one of the decoys gp[120] uses to evade the immune responses.[1,2] For example, variable loops can often adopt different conformations, and antibodies that recognize one conformation will not be able to effectively target another conformation.[3,4,5] Conformational masking can protect functionally conserved sites within gp[120]. The C4 region of gp[120], which consists of residues 416-4599 (HxB2 numbering10), has many important functional roles It is directly involved in receptor binding, co-receptor binding and co-receptor selection (tropism).[11,12] Crystal structures of gp[120] complexes have revealed that residues (Asn), (Met), and (Trp) in the C4 region have direct contact with CD4.13 The

Methods

Results

Conclusion