HIV-1 escape from VRC01-class broadly neutralizing antibodies (bNAbs) targeting the CD4-binding site (CD4bs) can proceed through mutations at envelope (Env) position 276 that eliminate a conserved N-linked glycosylation site, yet the effect of losing this glycan is paradoxically context-dependent: it increases resistance in some virus-antibody combinations and sensitivity in others. The source study (Murphy et al., Journal of Virology 2026) demonstrated this dual role across a 12-virus global Env panel and showed that static homology modeling cannot explain the observed functional variability. The open problem is to determine, functionally and structurally, how amino-acid identity at position 276 and the presence, absence, and processing state of the N276 glycan modulate neutralization of genetically diverse HIV-1 by VRC01-class and other CD4bs bNAbs, including the contribution of producer-cell-dependent glycan composition, and thereby to build a mechanistic account that explains the direction of these effects.

Background

The HIV-1 envelope glycoprotein (Env) is a trimer on the viral surface in which each protomer is cleaved into a gp120 surface subunit that binds host receptors and a gp41 subunit that drives membrane fusion; Env is the sole target of neutralizing antibodies. Within gp120, the CD4-binding site (CD4bs) is the conserved pocket that engages the host CD4 receptor to mediate cell entry; because it is functionally constrained, it is a major target of broadly neutralizing antibodies (bNAbs), antibodies that neutralize a wide range of viral strains. The best-studied CD4bs bNAbs are VRC01 and its class-mates (VRC07-523, 3BNC117, N6), all derived from the human immunoglobulin heavy-chain gene VH1-2 and mimicking CD4's mode of binding; they must negotiate the surrounding N-linked glycans (attached at asparagine positions 197, 276, 363, and 462/463). N-linked glycans are host-derived sugar chains that the cellular glycosylation machinery attaches at sequons (the amino-acid motif Asn-X-Ser/Thr, where X is any residue); on Env they shield epitopes from antibodies, aid folding, and can also form parts of bNAb epitopes. Position 276 lies in 'loop D' of gp120 at the periphery of the CD4bs and contacts the antibody light chain when VRC01-class bNAbs bind.

The N276 glycan has a well-documented dual character. In the donor from whom VRC01 was isolated, addition of a glycan at position 276 arose during co-evolution as an escape mutation that raised VRC01's required neutralizing concentration about tenfold (Lynch et al., 2015), and removal of glycan 276 generally increases viral sensitivity to VRC01-like bNAbs — so much so that CD4bs vaccine immunogens routinely delete N276 to expose the site to naive B cells (LaBranche et al., 2018; Agrawal et al., 2024). Conversely, for other CD4bs bNAbs such as 179NC75, the glycan itself is part of the epitope: its loss confers resistance and was observed as an in-vivo escape mutation in the 179NC75 donor (Freund et al., 2015). Cryo-electron microscopy structures of these 'glycan276-dependent' antibodies show that they retain the glycan in an unbound-like conformation, whereas VRC01 displaces it upon binding (Cottrell et al., 2021; the structural basis of VRC01 recognition itself is from Zhou et al., 2010).

Murphy et al. (2026) added a striking complication. Culturing infected CD4+ T cells under increasing VRC01 pressure, they evolved complete VRC01 resistance in a subtype A1C Env (246.F3) through an N276K mutation that destroys the glycosylation sequon — the 'wrong' direction given the literature above. Introducing N276D into the standardized 12-virus global Env reference panel (deCamp et al., 2014) and measuring neutralization (IC50, the concentration of antibody inhibiting infection by 50%, with IC80 the analogous 80%-inhibition concentration, in the standard TZM-bl reporter assay - TZM-bl being a HeLa-derived cell line engineered to carry the HIV-1 receptors CD4 and CCR5 plus a luciferase reporter, the field-standard neutralization platform) against a CD4bs bNAb panel, they found the effect of glycan-276 loss depends jointly on the Env background and the antibody: overall sensitivity increased for VRC01, VRC07-523, and N6, but several Envs (246.F3, 25710, CH119) became completely resistant to 3BNC117. Comparing N276D with N276K indicated that 3BNC117 depends more on the glycan itself while VRC01/VRC07-523 depend more on which residue is present, and producing viruses in Expi293F cells (which add more high-mannose, i.e., unprocessed, glycans, versus more complex processed glycans in HEK-293T) further shifted neutralization sensitivity. Static homology models (structures computed from a single related experimental template, without simulating molecular motion) of all 12 Env-VRC01 complexes showed nearly identical glycan-276 orientations and so failed entirely to explain this functional variability, indicating that dynamics, residue chemistry, or glycan conformational rearrangement governs the phenotype. Individual glycan sites of Env trimers are known to carry distinct and heterogeneous glycan compositions (Behrens et al., 2016), and computational glycan-topography approaches can design antigenicity changes at glycan positions (Yu et al., 2018), but neither has delivered a residue-resolved account of position 276. In vivo relevance is established: loss of the 276 glycosylation site (N276D) has been observed in trial participants with virologic rebound on bNAb therapy, and the site's context-dependent behavior is one reason genotypic prediction of CD4bs bNAb susceptibility remains only modestly accurate (Tao et al., 2026).

Problem Statement

Determine, functionally and structurally, how variation at HIV-1 Env position 276 modulates neutralization by CD4bs broadly neutralizing antibodies across genetically diverse virus, and why the effect is context-dependent. Concretely, as framed by Murphy et al. (2026), the deliverable is the mechanistic account defined at the end of this paragraph, and the following is a non-exhaustive program toward it rather than a checklist of independently sufficient parts: (1) measure the neutralization impact (e.g., IC50/IC80 in standard reporter assays) of different amino-acid substitutions at position 276 — all of which abolish the N276 sequon — together with matched variants that preserve Asn-276 but abolish (or restore) the sequon at the +2 Ser/Thr position — across a genetically diverse global panel of HIV-1 Envs, against a panel of CD4bs bNAbs including VRC01-class antibodies with differing light chains (e.g., VRC01, VRC07-523, 3BNC117, N6) and suitable non-CD4bs controls; (2) provide the structural and biophysical basis for the observed phenotypes — e.g., cryo-electron microscopy or crystal structures, nuclear magnetic resonance (NMR) spectroscopy, molecular-dynamics simulations (atomistic simulation of molecular motion), or site-specific glycan-composition mass spectrometry of matched Env-bNAb combinations — sufficient to distinguish the effect of the amino-acid residue itself (charge and size) from the effect of glycan loss and from the effect of glycan processing state, and to explain why glycan-276 loss confers resistance in some Env-bNAb combinations (such as 3BNC117 on 246.F3, 25710, CH119) while increasing sensitivity in others; and (3) define how producer-cell-dependent glycosylation (e.g., high-mannose-rich versus complex glycan profiles) contributes to these phenotypes, which is the in-vitro proxy for how host-environment glycosylation during natural infection would affect the same escape routes. The objective is a mechanistic account that explains the dual role of glycan N276 and why the direction of the neutralization-sensitivity change produced by a specified position-276 change depends on the Env background, the antibody, and the glycosylation state of the virus.

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