Structural flexibility at a major conserved antibody target on hepatitis C virus E2 antigen

Area of Science:

• Virology
• Immunology
• Structural Biology

Background:

• Hepatitis C virus (HCV) causes significant global liver disease.
• HCV envelope glycoproteins E1 and E2 are critical for viral entry.
• The E2 glycoprotein's CD81 receptor-binding site (CD81bs) is a target for broadly neutralizing antibodies (bNAbs).
• Current HCV vaccine strategies using recombinant E2 or E1E2 have limited success in eliciting protective bNAbs.

Purpose of the Study:

• To investigate the structural dynamics of the HCV E2 CD81bs.
• To understand why eliciting protective bNAb responses against the CD81bs is challenging.
• To inform rational vaccine design for improved HCV immunogens.

Main Methods:

• Electron microscopy (EM) to visualize antibody binding.
• Hydrogen-deuterium exchange (HDX) to assess protein flexibility.
• Molecular dynamics (MD) simulations to model protein motion.
• Calorimetry to determine protein stability.

Main Results:

• EM showed bNAb HCV1 binding to the E2 CD81bs from multiple angles, indicating flexibility.
• HDX and MD simulations confirmed significant flexibility of the entire CD81bs relative to the E2 core.
• Despite overall high protein stability (Tm = 84.8 °C), the CD81bs exhibits exceptional flexibility.
• This flexibility may favor non-neutralizing over broadly neutralizing antibody induction.

Conclusions:

• The HCV E2 CD81bs is highly flexible, posing a challenge for bNAb induction.
• Rigidifying the CD81bs is a necessary strategy for developing effective HCV vaccines.
• Understanding E2 dynamics is key to designing immunogens that elicit protective antibody responses.