Modulating the Antigenicity and Immunogenicity of HIV-1 Envelope by a Conserved N-Linked Glycan

Abstract

HIV-1 establishes persistent infection in part due to its ability to evade host immune responses. Occlusion by glycans contributes to masking conserved sites that are targets for some broadly neutralizing antibodies (bNAbs). Previous work has shown that removal of a highly conserved potential N-linked glycan (PNLG) site at amino acid residue 197 (N7) on the surface antigen gp120 of HIV-1 envelope glycoprotein (Env) increases neutralization sensitivity of the mutant virus to CD4 binding site (CD4bs)-directed antibodies and enhanced the ability of Env to generate cross-reactive neutralizing responses as compared to its WT counterpart. However, it is not clear if the role of the N7 glycan is conserved among diverse HIV-1 isolates and if other glycans in the conserved regions of HIV-1 envelope glycoprotein (Env) display similar functions. In this work, I examined the role of conserved PNLGs on the antigenicity of HIV-1 Env, particularly the role of the N7 glycan, in a panel of HIV-1 representing different clades, tissue origins, coreceptor usage, and neutralization sensitivity. I demonstrate that the absence of the N7 glycan increases the sensitivity of diverse HIV-1 isolates to CD4bs- and V3 loop-directed antibodies, indicating that the N7 glycan plays an important and conserved role modulating the structure, stability or accessibility of bNAb epitopes in the CD4bs and coreceptor-binding region, thus representing a potential target for immunogen design. I also examined if Tier 2 neutralizing antibodies and V1/V2 directed non-neutralizing antibodies previously correlated with protection in a clinical HIV-1 vaccine could be elicited by a poxvirus prime-gp120 boost strategy in a rabbit model using Env with or without the N7 glycan. I demonstrate that immunized rabbits generated cross-reactive neutralizing activities against >50% of Tier 2 global HIV-1 isolates tested in addition to V1/V2 binding antibodies. These findings demonstrate that antibody responses that have been correlated with protection against HIV-1 acquisition in humans can be elicited in a preclinical model by a poxvirus prime-gp120 boost strategy. However, the N7 glycan had little or no impact on Env immunogenicity in the context of the poxvirus prime-gp120 boost used, indicating the need for further improvements in immunization strategy by optimizing variables such as the nature of the priming and boosting immunogens. This work helps to define some of the parameters that may affect the antigenicity of Env and inform HIV-1 vaccine design.