Structural evidence for a bifurcated mode of action in the antibody-mediated neutralization of hepatitis C virus

Abstract

Hepatitis C virus (HCV) envelope glycoprotein E2 has been considered as a major target for vaccine design. Epitope II, mapped between residues 427-446 within the E2 protein, elicits antibodies that are either neutralizing or nonneutralizing. The fundamental mechanism of antibody-mediated neutralization at epitope II remains to be defined at the atomic level. Here we report the crystal structure of the epitope II peptide in complex with a monoclonal antibody (mAb#8) capable of neutralizing HCV. The complex structure revealed that this neutralizing antibody engages epitope II via interactions with both the C-terminal α-helix and the N-terminal loop using a bifurcated mode of action. Our structural insights into the key determinants for the antibody-mediated neutralization may contribute to the immune prophylaxis of HCV infection and the development of an effective HCV vaccine.

Fig. 1.

(A) Composite omit electron density map of mAb#8–epitope II complex at 2.85-Å resolution showing the epitope II peptide. The density is contoured at 2.0σ. (B) Surface representation of the mAb#8–epitope II complex structure. The CDR H1 loop is colored in bronze, H2 in pink, H3 in magenta, L1 in blue, L2 in teal, and L3 in cyan. The peptide is illustrated as a yellow cartoon. The CDR loops of mAb#8 form a half-circle-shaped groove to accommodate the peptide. The location of Gly⁴³⁶ is indicated by a red dot.

Fig. 2.

Interactions of mAb#8 with epitope II. The side chains of interacting residues are drawn in a ball-and-stick representation, with nitrogen atoms in blue, oxygen atoms in red, and sulfur atoms in light brown. Hydrogen bonds are indicated by red dotted lines. (A) Close-up view of the interactions of mAb#8 with the C-terminal portion of epitope II (⁴³⁷WLAGLF⁴⁴²). (B) Close-up view of the interactions of mAb#8 with the N-terminal portion of epitope II (⁴³⁰NESLNTG⁴³⁶).

Fig. 3.

Binding of mAb#8 to the epitope II peptide mutants in a competitive ELISA. The ELISA plate was coated with epitope II–long peptide. The antibody mAb#8 was incubated with different concentrations of a competitor peptide, as indicated on the X-axis: epitope II, epitope II Glu⁴³¹ > Ala, or epitope II Glu⁴³¹ > Ala, Asn⁴³⁴ > Glu before addition to the wells of the ELISA plate.