D108 Synthetic HIV-1 Glycopeptides Enable Characterization of Fine Epitopes of Broadly Neutralizing Antibody PG9

Abstract

Characterization of the precise epitopes for broadly neutralizing antibodies (bNAbs) is an important step for the design of an effective HIV-1 vaccine. Recent X-ray crystal structural studies indicate that the epitope of PG9 constitutes two N-glycans (at N160 and N156 positions) and a strand of peptide in the V1V2 domain of gp120. However, the precise nature of the neutralizing epitopes, particularly the fine structures of the N-glycans at N156 remains to be characterized. Further mapping of the epitopes is complicated by the complexity and heterogeneity of glycosylation of HIV-1 gp120. To address this problem, we launched a project aiming to identify the precise epitopes by synthesis and evaluation of a series of homogeneous cyclic glycopeptides corresponding to the V1/V2 domain. A chemoenzymatic method was developed to make such complex HIV-1 glycopeptides in which defined N-glycans were installed at the predetermined glycosylation sites. SPR binding studies with PG9 revealed that the fine structures of N-glycans at both the N156 and N160 sites were critical for the recognition of the synthetic antigens by PG9. In particular, a complex type sugar at the N156, rather than a Man5GlcNAc2 glycan, seems critical for the high affinity binding of PG9 to the antigen. The synthetic HIV-1 glycopeptides will be valuable to serve as coating antigens for detecting glycan-dependent neutralizing antibodies from sera of HIV-infected patients or those involved in HIV vaccine trials. These studies provide important insights for HIV-1 vaccine design.