SARS-CoV-2 Spike Peptide Mapping & Patient-Derived Neutralizing Antibody Discovery | AtaGenix

Based on: Li et al., Small Methods, 2021 (DOI: 10.1002/smtd.202100058)

Background

The SARS-CoV-2 spike (S) protein is the primary target of neutralizing antibodies and the basis of most COVID-19 vaccine strategies. Within its receptor-binding domain (RBD), the receptor-binding motif (RBM) directly contacts human ACE2 and is therefore a hotspot for protective immunity. But in 2021, the precise immunogenic epitopes within the spike — and which of those epitopes actually elicit neutralizing antibodies in patients — remained incompletely defined. Closing this gap was essential for rational vaccine design, diagnostic development, and therapeutic antibody discovery.

Approach

The research team designed a two-library strategy that combined epitope mapping and antibody discovery in a single integrated workflow:

• Peptide phage library: An overlapping peptide library covering the entire spike protein was constructed and screened against COVID-19 patient sera to identify which epitopes are recognized by the human immune system.
• ScFv phage library: A single-chain variable fragment (ScFv) library of 8.7×10⁹ clones was built from COVID-19 patient PBMCs, then panned against the identified epitopes to isolate human monoclonal antibodies directly from the patient repertoire.
• Validation: Structural bioinformatics, ELISA, pseudovirus neutralization assays, authentic virus neutralization, and mouse immunization confirmed the immunogenicity and protective potential of identified epitopes and antibodies.

Key Findings

1. Three Immunodominant RBM Epitopes

Approximately 15 peptides across the spike were recognized by patient sera, but three RBM epitopes stood out: S431–454, S470–486, and S501–515. All three are surface-exposed and divergent from SARS-CoV and MERS-CoV, indicating SARS-CoV-2 specificity.

Figure 1. Phage display peptide library screening identified ~15 immunogenic spike peptides. Three epitopes within the RBM (S431–454, S470–486, S501–515) showed the strongest and most consistent antibody reactivity across patient sera.

2. Diagnostic Potential (AUC ~0.98–1.0)

ELISA testing across 43 confirmed patients, 117 suspected patients, and 38 healthy controls showed that antibodies against these three peptides — particularly IgG — distinguished COVID-19 patients from healthy individuals with near-perfect sensitivity and specificity.

Figure 2. Antibody levels (IgM and IgG) against the three RBM peptides were significantly elevated in confirmed and suspected COVID-19 patients compared to healthy controls, demonstrating diagnostic utility with AUC values approaching 1.0.

3. S470–486: The Immunodominant Neutralizing Epitope

Synthetic peptides were tested for their ability to block viral entry. S470–486 inhibited pseudovirus infection in vitro, and — crucially — mice immunized with S470–486 generated neutralizing antibodies effective against authentic SARS-CoV-2. The other two epitopes (S431–454, S501–515) elicited binding antibodies but not neutralizing ones, establishing S470–486 as the immunodominant protective epitope within the RBM.

Figure 3. S470–486 blocked pseudovirus entry in vitro and elicited neutralizing antibodies in immunized mice, while S431–454 and S501–515 did not produce neutralizing responses — establishing S470–486 as the key protective epitope.

4. Patient-Derived Neutralizing Monoclonal Antibodies

From the 8.7×10⁹-clone ScFv phage library, two monoclonal antibodies targeting S470–486 were isolated. One clone (R3P1-F8) demonstrated potent neutralization against both pseudovirus and authentic SARS-CoV-2, proving that functional therapeutic antibodies can be rapidly obtained directly from patient immune repertoires using phage display — without the need for hybridoma or single B cell approaches.

Figure 4. Human monoclonal antibodies isolated from a COVID-19 patient ScFv phage library bound the S470–486 epitope and neutralized SARS-CoV-2 in both pseudovirus and authentic virus assays. Clone R3P1-F8 showed the strongest neutralization activity.

AtaGenix's Role

AtaGenix provided critical technical services that underpinned the discovery workflow:

• Phage display library construction: Both the overlapping peptide library (epitope mapping) and the patient-derived ScFv library (antibody discovery) leveraged AtaGenix's phage display platform, achieving the 10⁹-scale diversity required for comprehensive coverage.
• Library screening & panning: AtaGenix's biopanning protocols identified peptide-antibody interactions from complex patient sera, enabling the team to narrow from ~15 epitopes to the three most immunodominant targets.
• Antibody expression: Isolated ScFv clones were expressed and validated for binding and neutralization, confirming functional activity before downstream characterization.

Significance

This study demonstrated a complete workflow from epitope discovery to therapeutic antibody isolation in a single campaign: map immunogenic peptides → validate diagnostic utility → identify the neutralizing epitope → isolate patient-derived mAbs targeting that epitope. The S470–486 region emerged as a high-value target for both diagnostics and therapeutics. Critically, the entire discovery cycle was accelerated by phage display technology — bypassing the slower hybridoma and single B cell approaches — with AtaGenix's library construction and screening capabilities enabling the research team to achieve 10⁹-scale coverage that would have been difficult to build in an academic setting alone.

Reference

Li L, Gao M, Li J, et al. Methods to Identify Immunogenic Peptides in SARS-CoV-2 Spike and Protective Monoclonal Antibodies in COVID-19 Patients. Small Methods. 2021;5:2100058. DOI: 10.1002/smtd.202100058

This case study is based on a published research collaboration. Results may vary depending on target, library diversity, and patient sample quality. All proprietary client information is subject to NDA.