Targeting the Late Stage of HIV-1 Entry for Antibody-Dependent Cellular Cytotoxicity: Structural Basis for Env Epitopes in the C11 Region

Abstract

Summary Antibodies can impact HIV-1 infection in multiple ways including antibody-dependent cellular cytotoxicity (ADCC), a correlate of protection observed in the RV144 vaccine trial. One of the most potent ADCC-inducing epitopes on HIV-1 Env is recognized by the C11 antibody. Here we present the crystal structure, at 2.9 Å resolution, of the C11-like antibody N12-i3, in a quaternary complex with the HIV-1 gp120, a CD4-mimicking peptide M48U1, and an A32-like antibody, N5-i5. Antibody N12-i3 recognizes an epitope centered on the N-terminal “8th-strand” of a critical β-sandwich, which our analysis indicates to be emblematic of a late entry state, after the gp120 detachment. In prior entry states, this sandwich comprises only 7-strands, with the 8th strand instead pairing with a portion of the gp120 C terminus. The conformational gymnastics of HIV-1 gp120 thus includes altered β-strand pairing – possibly to reduce immunogenicity – though nevertheless still recognized by the human immune system.

Boosting with ALVAC-HIV and AIDSVAX B/E enhances Env constant region 1 and 2 antibody-dependent cellular cytotoxicity

“…Antigen complex structures of mAb A32 and N12-i3 (C11-like) [3, 6], antibodies isolated from HIV-1-infected individuals, confirm that DH677.3 recognized a unique epitope between the A32 and C11 antibody-binding sites involving Env epitope elements of both ( Fig 4 ). While the A32 antibody epitope consists exclusively of gp120 mobile layers 1 and 2 (76% and 24% of gp120 BSA, respectively; Table S4, Fig 4 B and C ), DH677.3 relies less on layers 1 and 2 (53% and 14% of gp120 BSA, respectively) and effectively utilizes the gp120 7-stranded β-sandwich (24% of gp120 BSA) ( Table S4, Fig 4 B and C ).…”

“…AIDSVAX B/E protein used in the RV144 and RV305 HIV-1 vaccine trial had an eleven amino acid N-terminal deletion [9] that removed a majority of the C11-like antibody epitope [6], whereas CRF_01 AE gp140 Env 92TH023 in ALVAC (vCP1521) did have the gp120 N-terminal 11 amino acids [10]. To determine if C11 could bind to gp120 proteins with an 11 amino acid N-terminal deletion, we assayed A32 and C11 antibodies for binding to full length AE.A244gp120 or to AE.A244gp120Δ11 (N-terminal 11 aa deleted).…”

“…The ability to recognize the 7-stranded β-sandwich renders DH677.3 similar to the C11-like antibody N12-i3, which almost exclusively depends on the β-sandwich for binding (94% of its total gp120 BSA; Table S4, Fig 4 B and C ). Interestingly, N12-i3 and other C11-like antibodies require the N-terminus of gp120 for binding and recognize a unique gp120 conformation formed by docking of the gp120 N-terminus as an 8 th strand to the β-sandwich to form an 8-stranded-β-sandwich structure [6]. The DH677.3 complex crystals were obtained with gp120 93TH057 core e which lacks the N-terminus (Δ11 aa deletion) and therefore the direct judgment, based on structure, whether or not the 8 th strand is involved in binding was not possible ( Fig 3 ).…”

“…C1C2-specific antibody epitopes have been termed Cluster A [1] and defined by two Env-targeted monoclonal antibodies (mAbs), A32 [2] and C11 [1]. Structural analyses of antigen complexes formed by A32, A32-like [3–5] and C11-like antibodies [6] indicate that these antibodies bind distinct Env epitopes. The A32 epitope involves a discontinuous sequence within layers 1 and 2 of the inner domain [4, 5] while the C11 epitope maps to the inner domain eight-stranded β sandwich [6].…”

“…Structural analyses of antigen complexes formed by A32, A32-like [3–5] and C11-like antibodies [6] indicate that these antibodies bind distinct Env epitopes. The A32 epitope involves a discontinuous sequence within layers 1 and 2 of the inner domain [4, 5] while the C11 epitope maps to the inner domain eight-stranded β sandwich [6]. Importantly, both antibodies are non-neutralizing for tier 2 HIV strains, but are capable of broad and potent ADCC [1, 2].…”