The HIV-1 envelope (Env) glycoprotein, a trimer of gp120-gp41 heterodimers, relies on conformational flexibility to function in fusing the viral and host membranes. Fusion is achieved after gp120 binds to CD4, the HIV-1 receptor, and a coreceptor, capturing an open conformational state in which the fusion machinery on gp41 gains access to the target cell membrane. In the well-characterized closed Env conformation, the gp120 V1V2 loops interact at the apex of the Env trimer. Less is known about the structure of the open CD4-bound state, in which the V1V2 loops must rearrange and separate to allow access to the coreceptor binding site. We identified two anti-HIV-1 antibodies, the coreceptor mimicking antibody 17b and the gp120-gp41 interface-spanning antibody 8ANC195, that can be added as Fabs to a soluble native-like Env trimer to stabilize it in a CD4-bound conformation. Here, we present an 8.9-Å cryo-electron microscopy structure of a BG505 Env-sCD4-17b-8ANC195 complex, which reveals large structural rearrangements in gp120, but small changes in gp41, compared with closed Env structures. The gp120 protomers are rotated and separated in the CD4-bound structure, and the three V1V2 loops are displaced by ∼40 Å from their positions at the trimer apex in closed Env to the sides of the trimer in positions adjacent to, and interacting with, the three bound CD4s. These results are relevant to understanding CD4-induced conformational changes leading to coreceptor binding and fusion, and HIV-1 Env conformational dynamics, and describe a target structure relevant to drug design and vaccine efforts.T he HIV-1 envelope (Env) glycoprotein, a trimer of gp120-gp41 heterodimers, mediates recognition of host receptors and fusion of the viral and target cell membranes (1). Structural flexibility of HIV-1 Env is required for its function in membrane fusion; thus, Env exists in multiple conformational states on the surface of virions (2). Fusion involves several steps: The gp120 portion of Env trimer first binds to the host receptor CD4 to capture a conformational state of Env that exposes the binding site for an HIV-1 coreceptor (CCR5 or CXCR4), which, in turn, leads to gp41-mediated fusion of the viral and host cell membranes. CD4-induced conformational changes within the Env trimer are incompletely understood. The binding of soluble CD4 (sCD4) produces little to no changes in the structures of gp120 cores (gp120 monomers with truncations in the N and C termini and variable V1V2 and V3 loops) (3), but results in rotation of the gp120 protomers within virion-bound Env trimers to create an open conformation distinct from the closed conformation of unliganded virion-bound trimers (4). Single-particle electron microscopy (EM) structures of recombinant native-like soluble Env gp140 trimers (SOSIPs) confirmed that they can adopt the same closed and open architectures as virion-bound Env trimers (5-7), thus the SOSIP substitutions (introduction of a disulfide bond linking gp120 to gp41 and an Ile→Pro mutation in gp41; ref. The closed co...

SummaryHIV-1 Env, a trimer of gp120-gp41 heterodimers, mediates membrane fusion after binding host receptor CD4. Receptor binding displaces V1V2 loops from Env's apex, allowing coreceptor binding and opening Env to enable gp41-mediated fusion. We present 3.54 Å and 4.06 Å cryoelectron microscopy structures of partially open soluble native-like Env trimers (SOSIPs) bound to CD4. One structure, a complex with a coreceptor-mimicking antibody that binds both CD4 and gp120, stabilizes the displaced V1V2 and reveals its structure. Comparing partially and fully open Envs with closed Envs shows that gp41 rearrangements are independent of the CD4-induced rearrangements that result in V1V2 displacement and formation of a 4-stranded bridging sheet. These findings suggest ordered conformational changes before coreceptor binding: (1) gp120 opening inducing side-chain rearrangements and a compact gp41 central helix conformation, and (2) 4-stranded bridging-sheet formation and V1V2 displacement. These analyses illuminate potential receptor-induced Env changes and inform design of therapeutics disrupting viral entry.

Summary Broadly neutralizing monoclonal antibodies protect against HIV-1 infection in animal models, suggesting that a vaccine that elicits them in humans would be effective. However, it has not yet been possible to elicit adequate serologic responses by vaccination. To activate B-cells expressing precursors of broadly neutralizing antibodies within polyclonal repertoires, we developed a new immunogen, RC1, which facilitates recognition of the V3-glycan patch on HIV-1 envelope while concealing non-conserved immunodominant regions by addition of glycans and/or multimerization on virus-like particles. Mouse, rabbit and rhesus macaque immunizations with RC1 elicited serologic responses targeting the V3-glycan patch. Antibody cloning and cryo-electron microscopy structures of antibody-envelope complexes confirmed that RC1 immunization expands clones of B-cells carrying anti-V3-glycan patch antibodies that resemble precursors of human broadly neutralizing antibodies. Thus, RC1 may be a suitable priming immunogen for sequential vaccination strategies to elicit V3-glycan antibodies in the context of polyclonal repertoires.

Some HIV-1–infected patients develop broad and potent HIV-1 neutralizing antibodies (bNAbs) that when passively transferred to mice or macaques can treat or prevent infection. However, bNAbs typically fail to neutralize coexisting autologous viruses due to antibody-mediated selection against sensitive viral strains. We describe an HIV-1 controller expressing HLA-B57*01 and HLA-B27*05 who maintained low viral loads for 30 years after infection and developed broad and potent serologic activity against HIV-1. Neutralization was attributed to three different bNAbs targeting non-overlapping sites on the HIV-1 envelope trimer (Env). One of the three, BG18, an antibody directed against the glycan-V3 portion of Env, is the most potent member of this class reported to date and, as revealed by crystallography and electron microscopy, recognizes HIV-1 Env in a manner that is distinct from other bNAbs in this class. Single-genome sequencing of HIV-1 from serum samples obtained over a period of 9 years showed a diverse group of circulating viruses, 88.5%(31 of 35) of which remained sensitive to at least one of the temporally coincident autologous bNAbs and the individual’s serum. Thus, bNAb-sensitive strains of HIV-1 coexist with potent neutralizing antibodies that target the virus and may contribute to control in this individual. When administered as a mix, the three bNAbs controlled viremia in HIV-1YU2–infected humanized mice. Our finding suggests that combinations of bNAbs may contribute to control of HIV-1 infection.

SUMMARY The HIV-1 envelope (Env) spike contains limited epitopes for broadly neutralizing antibodies (bNAbs); thus, most neutralizing antibodies are strain specific. The 8ANC195 epitope, defined by crystal and electron microscopy (EM) structures of bNAb 8ANC195 complexed with monomeric gp120 and trimeric Env, respectively, spans the gp120 and gp41 Env subunits. To investigate 8ANC195's gp41 epitope at higher resolution, we solved a 3.58Å crystal structure of 8ANC195 complexed with fully glycosylated Env trimer, revealing 8ANC195 insertion into a glycan shield gap to contact gp120 and gp41 glycans and protein residues. To determine whether 8ANC195 recognizes the CD4-bound open Env conformation that leads to co-receptor binding and fusion, one of several known conformations of virion-associated Env, we solved EM structures of an Env/CD4/CD4-induced antibody/8ANC195 complex. 8ANC195 binding partially closed the CD4-bound trimer, confirming structural plasticity of Env by revealing a previously unseen conformation. 8ANC195's ability to bind different Env conformations suggests advantages for potential therapeutic applications.