P19-03. Molecular mechanisms for enhancing the antigenicity of the carbohydrate epitope of the broadly neutralizing anti-HIV-1 antibody 2G12

Abstract

Background The broadly neutralizing antibody 2G12 is uniquely capable of recognizing the self-carbohydrate shield on the HIV-1 envelope glycoprotein gp120 and escaping immune tolerance. The preferred substrate of 2G12 is the Man4(D1)-arm of high-mannose glycans clustered on the gp120 outer surface. Remarkably, 2G12 has been shown to exhibit higher affinity for the non-self monosaccharide D-fructose than for D-mannose. D-fructose, in its pyranose form, resembles D-mannose, but with different substitutions at the anomeric and C-5 positions. These observations suggested other 'non-self' modifications to D-mannose may also enhance 2G12 binding. We consequently found that D-mannose with a C-6 methyl substitution binds 2G12 more strongly than both D-mannose and D-fructose. Moreover, introduction of this non-self modification to the terminus of the D1-arm creates the most potent monovalent 2G12 ligand known. We uncover here the molecular basis of this enhanced antigenicity, and the higher affinity of 2G12 for D-fructose over D-mannose, through high resolution crystallographic analyses.