Steric hindrance of antibody binding in an Omicron spike fusion intermediate.

Summary

This structural study shows that a conserved S2' proximal epitope (residues 815–825) becomes transiently accessible in an Omicron spike fusion intermediate but is sterically hindered for antibody engagement. These insights explain challenges in targeting this pan-coronavirus region and inform design of next-generation fusion-stage antibodies and vaccines.

Key Findings

• The pan-coronavirus S2' epitope (residues 815–825) is buried in prefusion spike but transiently exposed upon ACE2 binding.
• In the Omicron spike fusion intermediate, steric hindrance limits antibody engagement to this conserved epitope.
• Findings rationalize difficulties in eliciting broadly neutralizing responses to the S2' region and point to strategies for fusion-stage targeting.

Clinical Implications

While not immediately practice-changing, the work guides development of fusion-state antibodies or immunogens that can overcome transient exposure and steric hindrance at S2', potentially improving breadth against current and future coronaviruses.

Limitations

• Abstracted findings are structural and do not include in vivo neutralization or therapeutic efficacy data.
• Transient exposure may limit druggability; strategies to stabilize or access the epitope are not tested here.

Future Directions

Design and test fusion-stage immunogens and antibodies that can access S2' during transient exposure or stabilize the intermediate; validate breadth and efficacy in animal models.