Zachary A. Bornholdt, Hannah L. Turner, Charles D. Murin, Wen Li, Devin Sok, Colby A. Souders, Ashley E. Piper, Arthur Goff, Joshua D. Shamblin, Suzanne E. Wollen, Thomas R. Sprague, Marnie L. Fusco, Kathleen B. J. Pommert, Lisa A. Cavacini, Heidi L. Smith, Mark Klempner, Keith A. Reimann, Eric Krauland, Tillman U. Gerngross, Karl D. Wittrup, Erica Ollmann Saphire, Dennis R. Burton, Pamela J. Glass, Andrew B. Ward, Laura M. Walker

January 22, 2016

Overview

This study by Adimab and collaborating researchers is one of the earliest comprehensive profiling studies of the human B cell response to Ebola virus surface glycoprotein (EBOV GP), analyzing 349 GP-specific monoclonal antibodies (mAbs) from a convalescent donor who survived the 2014 Zaire EBOV outbreak. Of note, 77% of these antibodies effectively neutralized live EBOV, including several with exceptional potency. Structural and functional analyses revealed new sites of viral vulnerability, most notably in the GP stalk region near the viral membrane. These findings expand our understanding of protective humoral immunity to EBOV and provide a rational framework for advancing vaccine and antibody therapeutic design

Key hypotheses and objectives

The authors hypothesized that survivors of natural EBOV infection develop highly potent, diverse neutralizing antibodies that target previously uncharacterized epitopes on the virus’s surface glycoprotein. The study’s specific objectives were to:

• Assess the magnitude of the B cell response to EBOV GP and potency of the neutralizing antibody (Nab) response to GP.
• Define the clonal diversity, epitope representation, and mutation patterns within this donor’s EBOV GP-specific peripheral B cell repertoire.
• Identify the epitopes targeted by the mAbs responsible for neutralization across different domains of GP, including the glycan cap, GP1–GP2 interface, and stalk region.
• Determine the structural basis for neutralization and evaluate antibody performance for post-exposure therapeutic efficacy against lethal EBOV challenge using a murine model of infection.​

Experimental approach and techniques

The researchers collected peripheral memory B cells from a surviving convalescent donor three months after EBOV infection. Using fluorescently labeled EBOV GP constructs, GP-reactive B cells were isolated by FACS, generating 349 mAbs by single B cell cloning of immunoglobulin genes. Analysis of the VH and VL regions revealed that the anti-GP repertoire was highly diverse, containing 294 independent clonal lineages, skewed towards Vκ with longer CDRH3 lengths. 321 mAbs were systematical characterized by:

• Biolayer interferometry (BLI): Measurement of binding kinetics across GP variants, including the EBOV GP ectodomain, a mucin-like domain deletion construct, and a secreted GPI dimer GP isoform (sGP).
• Epitope competition assays: Grouping antibodies based on competition with reference mAbs, such as 1H3, 13C6 and KZ52, as well as high affinity mAbs from the panel.
• Neutralization assays: Quantifying antibody potency in vitro using live-virus plaque reduction (PRNT) assays.
• Single-particle electron microscopy (EM): Resolving antibody–GP complexes to identify binding footprints on GP’s three-dimensional crystal structure.
• Determination of Nab in vivo efficacy: Evaluation for post-exposure therapeutic efficacy against lethal EBOV challenge in a murine infection model.

Major findings and impact

The GP-specific antibody repertoire proved remarkably diverse, comprising 294 unique clonal lineages, almost all of which were somatically mutated. Most antibodies targeted GP epitopes that were not dependent on the native mucin-like domain, indicating immunodominance of exposed structural regions.

Epitope mapping by competition studies and electron microscopy revealed four major antigenic domains:

1. The glycan cap (targeted by 1H3 and 13C6) and sGP (which lacks the mucin-like domain but contains the GP1 core and glycan cap).
2. The GP1–GP2 interface, including epitopes overlapping with that of the reference mAb KZ52.
3. The GP stalk region, which includes the heptad repeat 2 (HR2) helix, proximal to the viral membrane, contained previously undescribed neutralizing epitopes on the GP1 head and HR2 stalk in addition to the previously known glycan cap and base regions. This revealed multiple, non-overlapping sites of vulnerability on Ebola GP.

In vitro assessment demonstrated that 77 and 63% of the mAbs reduced viral infectivity by 50 and 80% (PRNT₅₀ and PRNT₈₀), respectively, at concentrations ≤50 µg/ml, with several mAbs, including ADI-15758, ADI-15734, and ADI-15762, yielding PRNT₅₀ values of sub-0.05 μg/mL concentrations. Structural reconstructions demonstrated that stalk-binding antibodies recognized the membrane-proximal HR2 region inaccessible to most of the earlier therapeutic antibodies.

In a lethal EBOV challenge mouse model, antibodies targeting the GP1–GP2 interface and GP stalk yielded the highest survival rates (60–100%), outperforming a ZMapp component (mAb 2G4), which conferred only 40% protection. In contrast, glycan cap–directed mAbs generally lacked therapeutic efficacy when administered post-infection. Several human antibodies also conferred complete protection in mouse and guinea pig challenge models when administered post-exposure, marking the first demonstration of post-exposure protection using antibodies derived directly from a human survivor of EBOV.

Implications for therapeutic antibody development

This study substantially advanced understanding of EBOV-neutralizing antibody targeting mechanisms and provided critical design principles for next-generation countermeasures. Its key therapeutic implications include:

• Novel targets for immunotherapy: Antibodies binding the GP stalk and GP1–GP2 interface exhibit the most potent neutralization, suggesting these regions are priority targets for therapeutic and vaccine antigen engineering.
• Antibody cocktail optimization: By including stalk-targeting mAbs, future therapeutic cocktails could address viral escape that undermines existing antibody cocktails, such as MB-003 or ZMapp.
• Vaccine rational design: Insights into stalk-region immunogenicity support refining vaccine candidates to target exposed conserved neutralizing surfaces rather than immunodominant but nonprotective sites like the glycan cap.
• Early neutralizing response evidence: The presence of numerous potent clones just months after infection underscores that robust neutralizing immunity can develop early on, guiding expectations for vaccine-induced responses.

In summary, the authors provided an unprecedented panel of human anti-EBOV antibodies, highlighting a previously unknown stalk epitope with potential for therapeutic targeting. They demonstrated that protective efficacy is not always correlated with neutralization potency, indicating that some antibodies likely protect through Fc-mediated effector functions rather than direct viral neutralization. These findings marked a pivotal step toward engineering broad, potent Ebola immunotherapies and vaccines capable of targeting the virus’s most conserved and functionally critical regions.

For more details, read the full article in Science. 

Post-study note: Several antibodies characterized here informed the discovery paths leading to REGN-EB3 and mAb114, both of which later received FDA approval for Ebola virus disease following the 2018–2019 outbreak.