Regulation of epitope exposure in the gp41 membrane-proximal external region through interactions at the apex of HIV-1 Env

Abstract

Glycoprotein Env of human immunodeficiency virus type 1 (HIV-1) mediates viral entry through membrane fusion. Composed of gp120 and gp41 subunits arranged as a trimer-of-heterodimers, Env adopts a metastable, highly dynamic conformation on the virion surface. This structural plasticity limits the temporospatial exposure of many highly conserved, neutralizing epitopes, contributing to the difficulty in developing effective HIV-1 vaccines. Here, we employed antibody neutralization of HIV-1 infectivity to investigate how inter- and intra-gp120 interactions mediated by variable loops V1/V2 and V3 at the Env apex regulate accessibility of the gp41 membrane-proximal external region (MPER) at the Env base. Swapping the V3 loop from EnvSF162 into the EnvHXB2 background shifted MPER exposure from the prefusogenic state to a functional intermediate conformation that was distinct from the prehairpin-intermediate state sensitive to gp41-targeted fusion inhibitors. The V3-loop swap had a profound impact on global protein dynamics, biasing the equilibrium to a closed conformation resistant to most anti-gp120 antibodies, stabilizing the protein to both cold- and soluble CD4-induced Env inactivation, and increasing the CD4 requirements for viral entry. Further dissection of the EnvHXB2 V3 loop revealed that residue 306 uniquely modulated epitope exposure and trimer stability. The R306S substitution substantially decreased sensitivity to antibodies targeting the gp41 MPER and, surprisingly, the gp120 V3-loop crown (residues 312–315), but had only modest effects on exposure of intervening gp120 epitopes. Furthermore, the point mutation reduced soluble CD4-induced inactivation, but had no impact on cold inactivation. The residue appeared to exert its effects by electrostatically modifying the strength of intra-subunit interactions between the V1/V2 and V3 loops. The distinct patterns of neutralization and stability pointed to a novel prefusogenic Env conformation along the receptor activation pathway and suggested that apical Env-regulation of gp41 MPER exposure can be decoupled from much of the dynamics of gp120 subunits.

HIV-1 prehairpin intermediate inhibitors show efficacy independent of neutralization tier

“…Notably, the gp41-targeting bnAb 10E8v4 showed similarly narrow neutralization potencies as D5_AR and 5-Helix among tier-2/3 strains. The MPER is also preferentially exposed in the PHI conformation of gp41 ( 59 , 60 ), further corroborating the unique differences between gp41- and gp120-targeting inhibitors. Taken together, these results show that neutralization tiers have minimal predictive power for inhibition by 5-Helix and D5_AR IgG; further, such inhibitors show extremely narrow ranges of IC 50 values unlike best-in-class bnAbs currently employed in clinical studies.…”

Stable expression of HIV-1 MPER extended epitope on the surface of the recombinant probiotic bacteria Escherichia Coli Nissle 1917 using CRISPR/Cas9

“…The inclusion of the membrane milieu and surface presentation of immunogens would make it especially relevant for a HIV vaccine candidate [ 27 ]. The CRISPR/Cas9 gene editing system can be used for the insertion of heterologous sequences into bacterial chromosome with extremely high precision and efficiency [ 28 , 29 ].…”

Neutralization Sensitivity and Evolution of Virus in a Chronic HIV-1 Clade B Infected Patient with Neutralizing Activity against Membrane-Proximal External Region

“…Differences external to the MPER may affect the exposure and stability of the MPER, further affecting the neutralization sensitivity of the pseudovirus to mAbs [ 30 , 31 ]. The pseudoviruses 06–07, for which sequences were consistent with the consensus sequence in the MPER, were highly neutralization-sensitive to the bNAbs, and pseudovirus 06–14 was sensitive to 4E10, although it contained the 677R mutation.…”