Passively administered broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoprotein suppress viremia but select neutralization-resistant escape variants. Klein et al. (2014) showed in humanized mice that commonly arising, otherwise ineffective autologous and tier-1 neutralizing antibodies — particularly anti-V3-crown antibodies — can synergize with the V3-glycan bNAb 10-1074, because bNAb escape punches holes in the glycan shield that expose their epitopes, and asked whether HIV-1 can nevertheless escape both pressures, for example by shifting the N332 glycan to N334, without paying a fitness cost, and whether the antibodies present in most infected humans can partner therapeutic bNAbs in patients. Later work established much of the single-pressure picture: 10-1074 escape in humans proceeds through recurrent mutations at envelope positions 325/332/334 that carry measurable fitness costs, many high-mannose-patch bNAbs tolerate N332-to-N334 glycan shifts, and the randomized RIO trial (a placebo-controlled trial of the long-acting bNAbs 3BNC117-LS and 10-1074-LS given before analytical treatment interruption, the planned monitored pause of antiretroviral therapy) showed that reservoir sensitivity to autologous antibodies and to 10-1074 correlates with time to rebound, while rebound viruses are frequently resistant to both. The surviving open problem is the joint-selection question: quantifying whether glycan-shift and related V3-glycan-site escape routes permit durable evasion of combined bNAb and autologous antibody pressure at acceptable fitness cost, and defining the magnitude, breadth, and epitope specificity of autologous antibodies required for protective synergy in patients.

Background

The HIV-1 envelope glycoprotein (Env) spike carries a dense shield of N-linked glycans that conceals conserved protein surfaces from antibodies. One gap in this shield, the high-mannose patch (a cluster of immature high-mannose-type glycans) centered on the potential N-linked glycosylation site (PNGS, a short N-X-S/T amino-acid motif) at position 332 of the gp120 subunit — also called the V3-glycan supersite — is a major target of potent broadly neutralizing antibodies (bNAbs), antibodies that neutralize a wide range of viral strains, including 10-1074 and the PGT121/PGT128 families. Passively administered single bNAbs suppress viremia in animals and humans, but like single antiretroviral drugs they select neutralization-resistant escape variants.

Most infected individuals also mount strain-specific ('autologous') antibody responses that neutralize their own evolving virus but not unrelated strains. A large fraction of these commonly arising antibodies neutralize only 'tier-1' viruses — easy-to-neutralize laboratory-adapted or sensitive primary strains — whereas most circulating primary isolates are harder-to-neutralize 'tier-2' viruses. A classic observation (Wei et al., 2003) is that HIV-1 frequently escapes such antibodies not by mutating the antibody's contact residues but by adding or repositioning glycans (for example the N332 glycan shielding the V3 loop, the third variable loop of gp120, from anti-V3 antibodies), a route that costs the virus little replicative fitness.

Klein et al. (2014) connected these two threads. In humanized mice — immunodeficient mice reconstituted with human immune cells, which support HIV-1 infection but mount essentially no antibody response of their own — sequential therapy of HIV-1 YU2-infected animals with three bNAbs selected triple-escape mutants at no measurable fitness cost. Yet plasma from SHIV-AD8-infected macaques (SHIV is a chimeric simian immunodeficiency virus engineered to carry the HIV-1 envelope; macaques are an immunocompetent nonhuman primate model) and IgG from most HIV-1-infected humans contained commonly arising tier-1 antibodies, particularly antibodies binding the crown (tip) of the V3 loop, such as 10-188 and 1-79, that barely neutralized wild-type YU2 but potently neutralized the bNAb escape mutants: escaping a bNAb often opens a hole in the glycan shield that exposes otherwise hidden V3 epitopes. Combining 10-1074 with such a tier-1 antibody suppressed viremia in mice far longer than either antibody alone, with one combination preventing escape for six weeks. However, in 10-1074-treated macaques (Shingai et al., 2013) escape had occurred by removing the N332 PNGS while creating a new one at position 334 — a 'glycan shift' that evades the bNAb while presumably keeping V3 shielded from autologous antibodies — and the humanized-mouse system cannot model any contribution of host antibodies. Whether such joint escape is available to the virus at an acceptable fitness cost, and whether autologous antibodies can actually partner therapeutic bNAbs in patients, was left open.

Problem Statement

Determine whether HIV-1 can durably escape simultaneous selection by glycan-dependent V3-glycan broadly neutralizing antibodies (bNAbs, antibodies that neutralize a wide range of viral strains) and host autologous (tier-1/strain-specific) antibodies, and what this joint escape costs the virus. The problem has two linked components that together quantify the joint selection landscape governing bNAb-based immunotherapy:

(1) Glycan-shift escape routes. Establish whether PNGS shifts (relocation of a potential N-linked glycosylation site, the short N-X-S/T amino-acid motif that carries an N-linked glycan) such as N332 to N334 — and the alternative single mutations at envelope positions 332, 334, or 325 that remove or reposition the N332 glycan — allow the virus to durably evade both a glycan-dependent bNAb (10-1074/PGT121 class) and contemporaneous autologous antibody responses, while retaining envelope function, V3 shielding from those autologous antibodies, and in vivo replication fitness quantified relative to wild-type virus; the time horizon for durability and the acceptability criterion for the in vivo fitness cost must be stated and justified. Single-pressure experiments cannot settle this question; what is missing is direct phenotyping of glycan-shift variants against the contemporaneous autologous antibody repertoire together with measurement of their fitness.

(2) Protective correlates of autologous activity. Define the magnitude (titer), breadth, and epitope specificity of the autologous antibodies present in HIV-1-infected individuals that suffice, together with passively administered bNAbs, to delay or prevent escape.

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