Plasticity and Epitope Exposure of the HIV-1 Envelope Trimer

Abstract

We recently showed that mutations in the HIV-1 envelope (Env) destabilize the V3 loop, rendering neutralization-resistant viruses sensitive to V3-directed monoclonal antibodies (MAbs). Here, we investigated the propagation of this effect on other Env epitopes, with special emphasis on V2 loop exposure. Wild-type JR-FL and 19 mutant JR-FL pseudoviruses were tested for neutralization sensitivity to 21 MAbs specific for epitopes in V2, the CD4 binding site (CD4bs), and the CD4-induced (CD4i) region. Certain glycan mutants, mutations in the gp120 hydrophobic core, and mutations in residues involved in intraprotomer interactions exposed epitopes in the V2i region (which overlies the α4β7 integrin binding site) and the V3 crown, suggesting general destabilization of the distal region of the trimer apex. In contrast, other glycan mutants, mutations affecting interprotomer interactions, and mutations affecting the CD4bs exposed V3 but not V2i epitopes. These data indicate for the first time that V3 can move independently of V2, with V3 pivoting out from its "tucked" position in the trimer while apparently leaving the V2 apex intact. Notably, none of the mutations exposed V2 epitopes without also exposing V3, suggesting that movement of V2 releases V3. Most mutations increased sensitivity to CD4bs-directed MAbs without exposure of the CD4i epitope, implying these mutations facilitate the trimers' maintenance of an intermediate energy state between open and closed conformations. Taken together, these data indicate that several transient Env epitopes can be rendered more accessible to antibodies (Abs) via specific mutations, and this may facilitate the design of V1V2-targeting immunogens. Many epitopes of the HIV envelope (Env) spike are relatively inaccessible to antibodies (Abs) compared to their exposure in the open Env conformation induced by receptor binding. However, the reduced infection rate that resulted from the vaccine used in the RV144 HIV-1 vaccine trial was correlated with the elicitation of V2- and V3-directed antibodies. Previously, we identified various mechanisms responsible for destabilizing the V3 loop; here, we determined, via mutation of numerous Env residues, which of these elements maintain the V1V2 loop in an inaccessible state and which expose V1V2 and/or V3 epitopes. Notably, our data indicate that V3 can move independently of V2, but none of the mutations studied expose V2 epitopes without also exposing V3. Additionally, V1V2 can be rendered more accessible to Abs via specific mutations, facilitating the development of engineered V2 immunogens.

HIV-1 gp41 Residues Modulate CD4-Induced Conformational Changes in the Envelope Glycoprotein and Evolution of a Relaxed Conformation of gp120

“…5I and Table 3). Mutation L125F was previously reported to be an important determinant of intraprotomer interactions between V1/V2 and V3 in strain JR FL , allowing the exposure of both V2 and V3 neutralizing epitopes (42). Those studies also identified residue I184 as a potential interprotomer V1/V2 interaction partner with residue 165, consistent with our work identifying I165 as a key determinant of trimer opening in JR CSF Env.…”

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

“…In previous reports, I184 of one protomer has been shown to interact with L165 of the neighbouring protomer. This inter-protomeric interaction has been shown to be critical for quaternary interactions leading to the stabilization of the V1V2 regions of neighbouring protomers, and its loss has been shown to render JR-FL, a clade B HIV-1 strain, highly sensitive to V3 mAbs (42, 43). On similar lines, we observed L184 of one promoter interacting with R165 of another protomer [via van der Waals interactions between the solvent-accessible surface (SAS) of R165 and L184 on neighbouring protomers] ( Figure 8A ).The side chain of L184 was observed to be outward facing and did not make significant intra-protomeric interactions.…”

“…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.…”

Disruption of the HIV-1 Envelope allosteric network blocks CD4-induced rearrangements

“…Beginning from a closed state in which the trimer apex and topological layers surround the gp41 three-helix bundle 49 , engagement of a single CD4 induces β20-β21 loop rearrangement and is associated with Env triggering via residue I432 in β20-β21 and residue L193 in V1/V2. 30 Instability in V1/V2 associated with these changes would allow V3 exposure 34,50 and initial apex dissociation of a single gp120 protomer from the gp41 three-helix bundle 31 . Internal configurational changes in V1/V2 could then propagate to layers-1 and 2, thereby releasing W571, thus lowering the potential energy barrier to full gp120-gp41 three-helix bundle dissociation resulting in a single gp120 open, asymmetric trimer configuration.…”