Yeast-Elicited Cross-Reactive Antibodies to HIV Env Glycans Efficiently Neutralize Virions Expressing Exclusively High-Mannose N-Linked Glycans

Abstract

The HIV envelope (Env) protein uses a dense coat of glycans to mask conserved domains and evade host humoral immune responses. The broadly neutralizing antibody 2G12, which binds a specific cluster of high-mannose glycans on HIV Env, shows that the glycan shield can also serve as a target for neutralizing antibodies. We have described a triple mutant Saccharomyces cerevisiae strain that expresses high-mannose glycoproteins that bind to 2G12. When used to immunize rabbits, this yeast elicits antibodies that bind to gp120-associated glycans but fail to neutralize virus. Here we sought to determine the reason for these discordant results. Affinity purification of sera over columns conjugated with three 2G12-reactive yeast glycoproteins showed that these proteins could adsorb 80% of the antibodies that bind to gp120 glycans. Despite binding to monomeric gp120, these mannose-specific antibodies failed to bind cell surface-expressed trimeric Env. However, when Env was expressed in the presence of the mannosidase inhibitor kifunensine to force retention of high-mannose glycans at all sites, the purified antibodies gained the abilities to bind trimeric Env and to strongly and broadly neutralize viruses produced under these conditions. Combined, these data show that the triple mutant yeast strain elicits antibodies that bind to high-mannose glycans presented on the HIV envelope, but only when they are displayed in a manner not found on native Env trimers. This implies that the underlying structure of the protein scaffold used to present the high-mannose glycans may be critical to allow elicitation of antibodies that recognize trimeric Env and neutralize virus.

How Can HIV-Type-1-Env Immunogenicity Be Improved to Facilitate Antibody-Based Vaccine Development?

“…126 Unfortunately, these Abs do not neutralize HIV-1. 118,127 What, then, is the origin of the rare 2G12 MAb specificity that does efficiently neutralize many HIV-1 strains? 2G12 has as high an affinity for mannose-dependent epitopes on two species of the fungus Candida (albicans and tropicalis) as it does for gp120, an affinity that is unusually high for an anticarbohydrate Ab.…”

Combinations of Griffithsin with Other Carbohydrate-Binding Agents Demonstrate Superior Activity Against HIV Type 1, HIV Type 2, and Selected Carbohydrate-Binding Agent-Resistant HIV Type 1 Strains

“…10,11 The discovery that the monoclonal antibody (mAb) 2G12 targets N-linked glycans present on gp120 revealed that these glycans are interesting targets for neutralizing antibodies. 11,12 Antiretroviral drug resistance is one of the major reasons for anti-HIV therapy failure and the spread of resistant viral strains. To control and eventually diminish the HIV pandemic, novel prevention strategies are necessary.…”

Gold Nanoparticles Coated with Oligomannosides of HIV-1 Glycoprotein gp120 Mimic the Carbohydrate Epitope of Antibody 2G12

“…[27][28][29][30][31] Strategies that are not based on synthetic chemistry have also been proposed: Man 8 -reach mutant strains of a yeast glycoprotein elicited mannose-specific gp120-binding antibodies that anyway were practically unable to neutralize the virus, 32,33 unless virions forced to exclusively express high-mannose N-linked glycans were used. 34 The difficulty of eliciting high titers of antibodies to the 2G12 epitope remains a still-open challenge in search for carbohydrate-based vaccines against HIV.…”