The Conformational States of the HIV-1 Envelope Glycoproteins

Abstract

During HIV-1 entry into target cells, binding of the virus to host receptors, CD4 and CCR5/CXCR4, triggers serial conformational changes in the envelope glycoprotein (Env) trimer that result in the fusion of the viral and cell membranes. Recent discoveries have refined our knowledge of Env conformational states, allowing characterization of the targets of small-molecule HIV-1 entry inhibitors and neutralizing antibodies, and identifying a novel off-pathway conformation (State 2A). Here, we provide an overview of the current understanding of these conformational states, focusing on (i) the events during HIV-1 entry; (ii) conformational preferences of HIV-1 Env ligands; (iii) evasion of the host antibody response; and (iv) potential implications for therapy and prevention of HIV-1 infection.

Keywords: ADCC; HIV-1 Env; bNAbs; conformational states; entry inhibitors; vaccine design.

Figure 1.. Env conformational states during HIV-1 entry into cells.

The figure schematically depicts the HIV-1 Env conformational states relevant to the virus entry process. (A) The pretriggered (State-1) conformation resists the binding of all antibodies, except for most bNAbs and autologous (strain-restricted) neutralizing antibodies. (B) Binding to a single CD4 receptor triggers a transformation of the State-1 Env to a default intermediate, partially “open” conformation (State 2). (C) In the full CD4-bound conformation (State 3), the gp41 heptad repeat (HR1) region coiled coil is formed and exposed. This pre-hairpin intermediate is competent for interaction with the CCR5 coreceptor, which promotes additional conformational changes in Env. (D) As a result, the hydrophobic fusion peptide at the N-terminus of gp41 is inserted into the target cell membrane and the energetically stable six-helix bundle (6HB) is formed, driving the fusion of the viral and cell membranes.

Figure 2.. Antibody-mediated antiviral mechanisms and HIV-1 evasion strategies.

(A) Antiviral mechanisms involving antibodies against Env are depicted. State-1 Envs on the surface of primary HIV-1 can be bound by bNAbs, resulting in virus neutralization. Non-neutralizing antibodies (Non-NAbs) can bind and neutralize HIV-1 only if Env samples downstream conformations. Tier 1 Envs spontaneously sample these conformations. Alternatively, membrane-bound CD4 or CD4mc can stabilize these more “open” conformations. Various scenarios leading to the elimination of infected cells by ADCC are depicted. The binding of some bNAbs to cell-surface Env can result in ADCC. The Cluster A epitope on the gp120 inner domain is a target for potent ADCC-mediating antibodies and is displayed in an asymmetric, “partially open” Env conformation (State 2A). State 2A can be induced by a combination of a CD4mc and a CD4i (anti-CoRBS) antibody, or by the interaction of Env and CD4 on the same membrane. (B) Strategies used by HIV-1 to evade inhibition by antibodies are depicted. Viruses accumulate changes in Env sequence, allowing escape from bNAbs. HIV-1 accessory proteins decrease the effectiveness of ADCC. Nef prevents the expression of NKG2D ligands (NKG2DL), thus inhibiting NK cell activation. CD4 expression is reduced by Vpu, Nef and Env; as a result, the CD4i epitopes of Env are no longer triggered by membrane-bound CD4 and the infected cells remain resistant to ADCC mediated by CD4i Abs. Vpu also protects infected cells from ADCC responses by allowing efficient viral release, thus diminishing the number of Env antigens on the cell surface.

Figure 3.. Env conformation: implications for HIV-1 prophylaxis.

(A) Envs on primary HIV-1 viral particles are predominantly in a State-1 conformation, which presumably elicits bNAbs in approximately 10% of HIV-1-infected humans. (B,C) Soluble gp140 SOSIP.664 trimers are stabilized in a State-2 conformation, and induce autologous Tier-2 neutralizing Abs in guinea pigs, rabbits and monkeys. They also elicit cross-reactive Tier-2 neutralizing Abs in cows. (D) Methods to stabilize Env in State-1 may potentially increase the efficiency of bNAb generation in humans. One such method of stabilizing State-1 conformations is the use of small-molecule conformational blockers like BMS-806. (E) CD4mcs, such as BNM-III-170, induce State 2/3 Env conformations. State 2/3 Env immunogens could raise antibodies that neutralize viral particles exposed to CD4mc and mediate ADCC against CD4mc-treated, HIV-1-infected cells in the course of treatment or prophylaxis.