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Changes in structure and antigenicity of HIV-1 Env trimers resulting from removal of a conserved CD4 binding site-proximal glycan.

Changes in structure and antigenicity of HIV-1 Env trimers resulting from removal of a conserved CD4 binding site-proximal glycan.
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Liang Y, Guttman M, Williams JA, Verkerke H, Alvarado D, Hu SL, Lee KK,


Liang Y, Guttman M, Williams JA, Verkerke H, Alvarado D, Hu SL, Lee KK, (click to view)

Liang Y, Guttman M, Williams JA, Verkerke H, Alvarado D, Hu SL, Lee KK,

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Journal of virology 2016 8 3() pii

Abstract

The envelope glycoprotein (Env) is the major target for HIV-1 broadly neutralizing antibodies (bNAbs). One of the mechanisms HIV has evolved to escape the host’s immune response is to mask conserved epitopes on Env with dense glycosylation. Previous studies have shown that removal of a particular conserved glycan at N197 increases neutralization sensitivity of the virus to antibodies targeting the CD4 binding site (CD4bs), making it a site of significant interest from the perspective of vaccine design. At present, the structural consequences that result from removal of the N197 glycan have not been characterized. Using native-like SOSIP trimers, we examine the effects on the antigenicity and local structural dynamics resulting from removal of this glycan. A large increase in the binding of CD4bs and V3-targeting antibodies is observed in the N197Q mutant in trimeric Env while no changes are observed with monomeric gp120. While the overall structure and thermo-stability are not altered, a subtle increase in flexibility of the variable loops at the trimeric interface of adjacent protomers is evident in the N197Q mutant by Hydrogen/Deuterium-exchange mass spectrometry. Structural modelling of the glycan chains suggests that the spatial occupancy of the N197 glycan leads to steric clashes with CD4bs antibodies in the Env trimer, but not monomeric gp120. Our results indicate that removal of the N197 glycan enhances exposure of relevant bNAb epitopes on Env with a minimal impact on the overall trimeric structure. These findings present a simple modification for enhancing trimeric Env immunogens in vaccines.

IMPORTANCE
The HIV-1 Env glycoprotein presents a dense patchwork of host-cell-derived N-linked glycans. This so-called glycan shield is considered to be a major protective mechanism from immune recognition. While positions of many N-linked glycans are isolate-specific, some are highly conserved and are believed to play key functional roles. In this study, we examine the conserved, CD4 binding site-proximal N197 glycan and demonstrate that its removal both facilitates neutralizing antibody access to the CD4 binding site and modestly impacts the structural dynamics at the trimer crown, without drastically altering global Env trimer stability. This indicates that surgical glycosylation site modification may be an effective way of sculpting epitope presentation in Env-based vaccines.

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