Structural Constraints at the Trimer Apex Stabilize the HIV-1 Envelope in a Closed, Antibody-Protected Conformation

Abstract

The human immunodeficiency virus type 1 (HIV-1) envelope (Env) trimer evades antibody recognition by adopting a closed prefusion conformation. Here, we show that two conserved tyrosines (Y173, Y177) within the second variable (V2) loop of the gp120 Env glycoprotein are key regulators of the closed, antibody-protected state of the trimer by establishing intramolecular interaction with the base of the third variable (V3) loop. Mutation of Y177 and/or Y173 to phenylalanine or alanine dramatically altered the susceptibility of diverse HIV-1 strains to neutralization, increasing sensitivity to weakly and nonneutralizing antibodies directed against diverse Env regions, consistent with the adoption of an open trimer configuration. Conversely, potent broadly neutralizing antibodies (bNAbs) against different supersites of HIV-1 vulnerability exhibited reduced potency against V2 loop tyrosine mutants, consistent with their preferential targeting of the closed trimer. Mutation of V3 loop residues predicted to interact with the V2 loop tyrosines yielded a similar neutralization phenotype. Sera from chronically HIV-1-infected patients contained very high titers of antibodies capable of neutralizing V2 loop tyrosine mutants but not wild-type viruses, indicating that the bulk of antibodies produced in infected hosts are unable to penetrate the protective shield of the closed trimer. These results identify the tyrosine-mediated V2-V3 loop complex at the trimer apex as a key structural constraint that facilitates HIV-1 evasion from the bulk of host antibodies. IMPORTANCE The extraordinary ability of human immunodeficiency virus type 1 (HIV-1) to evade host immunity represents a major obstacle to the development of a protective vaccine. Thus, elucidating the mechanisms whereby HIV-1 protects its external envelope (Env), which is the sole target of virus-neutralizing antibodies, is an essential step toward vaccine design. We identified a key structural element that maintains the HIV-1 Env trimer in a closed, antibody-resistant conformation. A major role is played by two conserved tyrosines at the apex of the Env spike, whose mutation causes a global opening of the trimer structure, exposing multiple concealed targets for neutralizing antibodies. We also found that HIV-infected individuals produce very large amounts of antibodies that neutralize the open Env form; however, the bulk of these antibodies are unable to penetrate the tight defensive shield of the native virus. This work may help to devise new strategies to overcome the viral defensive mechanisms and facilitate the development of an effective HIV-1 vaccine.

Flow Virometry Quantification of Host Proteins on the Surface of HIV-1 Pseudovirus Particles

“…Despite decades of research, the human immunodeficiency virus (HIV) remains a prominent global issue, with 38 million people estimated to be living with the virus in 2019 [ 1 ]. Targeting the viral envelope glycoprotein, gp120, in vaccine designs has proven to be difficult due to the protein’s extensive glycosylation, frequent mutations, and conformational camouflage of neutralizing epitopes [ 2 , 3 , 4 , 5 , 6 ]. A less commonly discussed feature of the viral envelope that may be of interest when informing new therapeutics or vaccine designs is the myriad of cellular proteins embedded within it.…”

A rare mutation in an infant derived HIV-1 envelope glycoprotein alters interprotomer stability and susceptibility to broadly neutralizing antibodies targeting the trimer apex

“…As V2-apex bnAbs target the closed conformation of the trimer, based on neutralization profile observed, we hypothesized that the most plausible reason for the loss of susceptibility to V2-apex bnAbs was the loss of closed conformation of the Env. In its closed conformation, the Env trimer occludes several immunodominant epitopes that are targeted by non-neutralizing antibodies (non-nAbs) (39, 41). When neutralization profile against several non-nAbs (those targeting CD4-induced epitopes and V3 loop) was assessed, the L184F mutant virus showed relatively higher neutralization susceptibility to non-nAbs.…”

“…Substitution of the small side chain of leucine with the bulky non-polar side chain of phenylalanine led to disruption of interprotomer interactions that have been observed to be critical for maintenance of the trimeric apex. Mutations that alter or disrupt interprotomer contacts in Env trimer have been shown to change its susceptibility to several classes of bnAbs (39, 41–43). Overall, our results provide information on the role of a rare escape mutation L184F in the viral envelope that led to resistance to bnAbs targeting the V2-apex.…”

Quantifying Anti-HIV Envelope-Specific Antibodies in Plasma from HIV Infected Individuals

“…Unliganded Env is normally present in a closed conformation on HIV-infected cells [15]. Advancements in electron microscopy and cryo-tomography have shown that highly conserved epitopes are hidden in the native Env trimer [13,48,49,50,51].…”