Prefusion-specific glycoprotein B human antibodies protect against neonatal HSV-2 infection

By utilizing prefusion-stabilized HSV-2 glycoprotein B and LIBRA-seq technology, researchers identified four novel human antibodies that bind unique epitopes and provide potent cross-neutralization and protection against neonatal HSV-2 infection in mice.

The Gist

The Big Picture: A New Shield Against a Sneaky Virus

Imagine Herpes Simplex Virus (HSV) as a master thief that breaks into your body and sets up a permanent, uninvited camp in your nervous system. While it often just causes annoying cold sores or genital lesions, it can be deadly for newborns if passed from mother to baby during birth. Currently, we have drugs to slow the thief down, but no vaccine to stop them from entering, and the drugs don't always work perfectly.

The virus has a "key" it uses to unlock and enter your cells. This key is a protein called Glycoprotein B (gB). Think of gB as a spring-loaded trapdoor. To break in, the virus has to snap this trapdoor from a "locked" state (prefusion) into an "open" state (postfusion).

The Problem: Scientists have been trying to build antibodies (our body's security guards) to stop this trapdoor. But most of the guards we've found so far only recognize the trapdoor after it has already snapped open. By then, the virus is already inside the house. We needed guards that could recognize the trapdoor while it was still locked and ready to spring, but until now, we couldn't find any human antibodies that could do that.

The Breakthrough: Finding the "Prefusion" Guards

This study is like a high-tech treasure hunt. The researchers used a special tool called LIBRA-seq (think of it as a super-powered metal detector) to scan the blood of people who had already been infected with HSV. They were looking for specific security guards (B-cells) that could recognize the virus's "locked" trapdoor.

They found four special human antibodies that act like elite snipers. These guards don't wait for the door to open; they spot the virus the moment it tries to approach and lock the trapdoor in place, preventing the virus from ever entering the cell.

The Two Super-Guards: 5-18 and 3-6

Out of the four guards found, two stood out as the absolute champions: Antibody 5-18 and Antibody 3-6.

1. Antibody 5-18 (The "Spanner"): Imagine the virus's trapdoor is made of two separate pieces of wood (domains) that are hinged together. Antibody 5-18 is like a heavy-duty wrench that grabs both pieces of wood at the same time and jams them together. It bridges the gap between them, making it physically impossible for the door to snap open.
2. Antibody 3-6 (The "Hinge Jammer"): This antibody is like a rusty nail driven right into the hinge of the trapdoor. It wedges itself into the flexible joint where the door bends. When the virus tries to bend the door to open it, the nail stops the movement, freezing the virus in place.

Why is this cool?

• Cross-Protection: These guards work against both HSV-1 (cold sores) and HSV-2 (genital herpes). It's like having one master key that locks both your front and back doors.
• Baby Protection: The researchers tested these antibodies on newborn mice (a model for human babies). When the mice were exposed to a lethal dose of the virus, the mice treated with these antibodies survived at very high rates (up to 91% for antibody 3-6). This is huge because it suggests these antibodies could save newborn lives.

The "Why Didn't We Find This Before?" Mystery

You might wonder, "If these guards are so good, why didn't we find them earlier?"

The paper explains that the virus is a master of disguise.

• The Shape-Shifter: The "locked" version of the trapdoor (prefusion) looks very similar to the "open" version (postfusion) in many ways. Most of our immune system's training focuses on the "open" version because that's what the virus looks like after it's already done its job.
• The Hidden Spot: The specific spots these new antibodies lock onto are like secret compartments that only exist when the trapdoor is locked. Once the door snaps open, those spots disappear or move too far apart. Our immune system usually ignores these secret spots because they are hard to see or because the virus hides them with a "glycan shield" (a layer of sugar molecules that acts like camouflage).

However, the researchers found that on the HSV-2 virus, these secret spots are actually exposed and vulnerable, waiting for a guard to find them.

The Future: A New Weapon for Newborns

This research is a game-changer for two reasons:

1. Proof of Concept: It proves that humans can naturally make antibodies that target the virus's most vulnerable, pre-attack state. This gives vaccine developers a clear target: design a vaccine that teaches the body to make these specific "trapdoor-jamming" guards.
2. Immediate Hope: These antibodies (especially 3-6) are potent enough to be used as a treatment. In the future, doctors might be able to give a dose of these antibodies to a mother or a newborn immediately after birth to prevent the virus from taking hold, acting as a temporary shield until the baby's own immune system can handle it.

The Bottom Line

Think of this study as finding the perfect lockpick for a master thief's door. For years, we tried to stop the thief after he broke in. Now, we have found a way to jam the door shut before he even touches the handle. This discovery brings us one giant step closer to finally protecting the most vulnerable among us—our newborns—from a virus that has plagued humanity for centuries.

Technical Summary

1. Problem Statement

• Clinical Burden: Herpes Simplex Virus (HSV), particularly HSV-2, causes severe neonatal infections with high mortality rates (~30% for disseminated disease) and long-term neurological sequelae. Current treatments (acyclovir) offer limited efficacy, and no approved vaccines exist.
• Therapeutic Gap: While monoclonal antibodies (mAbs) targeting viral glycoproteins are a promising therapeutic avenue, most existing neutralizing antibodies target Glycoprotein D (gD). Antibodies targeting Glycoprotein B (gB) are scarce, and critically, no human antibodies targeting the prefusion conformation of gB had been identified.
• Structural Challenge: gB is a Class III fusion protein that undergoes a massive conformational change from a metastable prefusion state to a stable postfusion state during viral entry. The prefusion state exposes unique, vulnerable epitopes that are often obscured or disrupted in the postfusion state. Stabilizing gB in its prefusion state has been technically difficult, hindering the discovery of antibodies that target these specific sites.

2. Methodology

The study employed a multi-disciplinary approach combining high-throughput B-cell sequencing, structural biology, and in vivo challenge models:

• Antigen Design: The researchers utilized prefusion-stabilized HSV-2 gB constructs (specifically the G3 variant) alongside postfusion constructs. They also included stabilized gB from other herpesviruses (HCMV, EBV) and control antigens (HIV, Influenza) to filter non-specific binders.
• LIBRA-seq (Linking B cell Receptor Sequence to Antigen Specificity through Sequencing):
  • Peripheral Blood Mononuclear Cells (PBMCs) were collected from HSV-2 seropositive donors and healthy controls.
  • Cells were stained with DNA-barcoded prefusion and postfusion gB antigens.
  • Single-cell sequencing was performed to link B-cell receptor (BCR) sequences to antigen specificity.
  • A LIBRA-seq score (LSS) was calculated to identify B cells with high specificity for prefusion gB.
• Antibody Screening & Characterization:
  • Selected B-cell clones were recombinantly expressed as IgG1 mAbs.
  • Binding Specificity: Indirect ELISA and Biolayer Interferometry (BLI) were used to assess binding to prefusion vs. postfusion gB and to determine kinetic parameters ($K_d$, $k_{on}$, $k_{off}$).
  • Neutralization: Plaque-reduction assays were conducted against HSV-2 (strain G) and HSV-1 (strain NS) clinical isolates to determine IC50 values.
  • Autoreactivity: Flow cytometry was used to test for binding to human epithelial cell lines (HEp-2, HEK293T) to rule out autoimmunity.
• Structural Biology (Cryo-EM):
  • Cryo-electron microscopy was used to solve the structures of the prefusion-stabilized HSV-2 gB ectodomain in complex with the Fab fragments of the top candidate antibodies (5-18, 1-14, 3-6, and 3-5).
  • Structures were refined to resolutions ranging from 2.9 Å to 3.2 Å to map epitopes and understand the mechanism of prefusion specificity.
• In Vivo Efficacy:
  • A neonatal mouse model (2-day-old C57BL/6J pups) was used.
  • Pups were treated intraperitoneally with mAbs and immediately challenged intranasally with a lethal dose of HSV-2 (CNS11 strain).
  • Survival rates were monitored for 21 days.

3. Key Contributions

• Discovery of Human Prefusion-Specific Antibodies: This is the first report of human-derived antibodies that specifically recognize the prefusion conformation of HSV-2 gB. Previously, only camelid nanobodies and murine antibodies targeting this state had been described.
• Identification of Novel Epitopes: The study mapped four distinct prefusion-specific epitopes:
  • Quaternary Epitopes (5-18, 1-14): Spanning domains I and IV of adjacent protomers.
  • Interdomain Epitope (3-6): Spanning domains I, II, and the hinge region within a single protomer.
  • Membrane-Proximal Epitope (3-5): Located at the tip of domain I (though this antibody showed poor neutralization due to steric hindrance on native virions).
• Structural Basis for Specificity: Cryo-EM structures revealed exactly how these antibodies lock gB in the prefusion state or recognize conformational changes that occur during fusion, explaining why they do not bind the postfusion form.
• Cross-Neutralization: Demonstrated that human antibodies can potently neutralize both HSV-1 and HSV-2, a critical finding given the clinical overlap of these viruses.

4. Key Results

• Antibody Isolation: From 449 antigen-specific B cells, four prefusion-specific (5-18, 1-14, 3-6, 3-5) and two prefusion-preferring antibodies were identified.
• Binding Kinetics:
  • Antibodies 5-18, 1-14, and 3-6 showed negligible binding to postfusion gB but high-affinity binding to prefusion gB (picomolar $K_d$).
  • 3-6 exhibited the highest affinity for HSV-2 gB ($K_d$ = 0.12 nM) and improved affinity for HSV-1 gB ($K_d$ = 0.08 nM).
• Neutralization Potency:
  • mAb 3-6 was the most potent neutralizer against both HSV-2 ($IC_{50}$ = 0.29 nM) and HSV-1 ($IC_{50}$ = 0.34 nM), outperforming the well-characterized anti-gB antibody D48 and the anti-gD antibody HSV8.
  • mAb 5-18 also showed strong cross-neutralization ($IC_{50}$ = 1.25 nM for HSV-2; 3.30 nM for HSV-1).
  • mAb 3-5, despite high binding affinity, showed poor neutralization, likely because its epitope is sterically occluded by the viral membrane or fusion loops in the native virion context.
• Structural Insights:
  • 5-18 and 1-14 bind quaternary epitopes across adjacent protomers (Domains I and IV), effectively "locking" the trimer and preventing the conformational rearrangement required for fusion.
  • 3-6 binds a unique epitope involving the hinge region between Domains I and II. Structural superposition showed that the postfusion conformation causes a reorientation of Domain II that would sterically clash with Fab 3-6.
• In Vivo Protection:
  • In the neonatal mouse model, mAb 3-6 provided 91% survival, which was statistically equivalent to the gold-standard anti-gD antibody HSV8 (91%) and superior to the anti-gB control D48 (87%).
  • mAb 5-18 provided 75% survival.
  • mAb 1-14 provided limited protection (20%), suggesting that epitope accessibility or orientation on the native virion is crucial for efficacy.
• Conservation Analysis: The epitopes targeted by 5-18 and 3-6 are highly divergent in other human herpesviruses (HCMV, EBV), indicating that these antibodies are specific to HSV and unlikely to provide pan-herpesvirus protection. However, the lack of glycan shielding on the HSV-2 epitopes makes them accessible targets.

5. Significance

• Therapeutic Potential: The study validates prefusion-stabilized gB as a viable target for human antibody therapeutics. The high potency of mAb 3-6 in a lethal neonatal model suggests it could be developed as a passive immunotherapy to prevent neonatal HSV-2 infection, a condition with currently high mortality.
• Vaccine Design: The identification of these vulnerable, prefusion-specific epitopes provides a structural blueprint for designing next-generation HSV vaccines. Vaccines incorporating prefusion-stabilized gB could potentially elicit similar potent, neutralizing antibody responses in humans.
• Mechanistic Understanding: The work elucidates the structural mechanisms by which human antibodies recognize and neutralize Class III fusion proteins, bridging a gap between viral entry mechanisms and immune recognition.
• Proof of Concept: It demonstrates that LIBRA-seq combined with stabilized antigens is a powerful strategy for isolating rare, high-affinity human antibodies against conformationally complex viral targets.

In conclusion, this paper represents a major breakthrough in HSV research by identifying the first human antibodies that specifically target the prefusion state of gB, demonstrating their potent cross-neutralizing activity, and proving their efficacy in protecting neonates from lethal infection.