From Structural Studies to HCV Vaccine Design

Abstract

Hepatitis C virus (HCV) is a serious and growing public health problem despite recent developments of antiviral therapeutics. To achieve global elimination of HCV, an effective cross-genotype vaccine is needed. The failure of previous vaccination trials to elicit an effective cross-reactive immune response demands better vaccine antigens to induce a potent cross-neutralizing response to improve vaccine efficacy. HCV E1 and E2 envelope (Env) glycoproteins are the main targets for neutralizing antibodies (nAbs), which aid in HCV clearance and protection. Therefore, a molecular-level understanding of the nAb responses against HCV is imperative for the rational design of cross-genotype vaccine antigens. Here we summarize the recent advances in structural studies of HCV Env and Env-nAb complexes and how they improve our understanding of immune recognition of HCV. We review the structural data defining HCV neutralization epitopes and conformational plasticity of the Env proteins, and the knowledge applicable to rational vaccine design.

Keywords: E1; E1E2 complex; E2; VH1-69; envelope glycoproteins; hepatitis C virus (HCV); neutralization face; neutralizing antibodies; structural studies; vaccine design.

Figure 1

HCV E1 and E2 Env proteins. Schematic representation of the HCV E1 and E2 Env proteins and E2 core domains that were used for E2 structural studies, colored by structural component. Numbering is based on the H77 prototypic strain. N-linked glycans are shown in green, and the conserved cysteines in blue. For E2, disulfide bonds, based on the H77 E2C-AR3C structure, are shown in blue dashed lines. NTD, N’ terminal domain; pFP, putative fusion peptide; CR, conserved region; TMD, transmembrane domain; HVR1, hypervariable region 1; AS412, antigenic site 412; VR, variable region; CD81 bl, CD81 binding loop; TM, transmembrane.

Figure 2

The structure of E2 Env. (a) The structure of H77 E2c (PDB: 4MWF) with the molecular surface color-coded as in Figure 1. The structures of E2 core domain reveal a novel protein fold consisting of a central immunoglobulin (Ig) β-sandwich fold (red), which is stabilized by the conserved disulfide bonds and flanked by an N-terminal front layer (cyan) and a C-terminal back layer (green). The long CD81 binding loop (blue) extends from the central β-sandwich domain. (b) E2 neutralization antigenic sites mapped onto the H77 E2c surface. The AR1 immune decoy site is also shown. AR3 is marked in dashed line, and AS412, AS434, and AR1 are colored in pink, yellow, and green, respectively. The mAbs that were used for structural studies and epitope characterization are listed.

Figure 3

The binding mode of AR3-targeted V_H1-69-encoded bnAbs. (a) Structural comparison of the epitopes and the CDR positions of the bnAb subgroups. AR3A is representative of the AR3A/B/C/D, U1 and HC11 group; HEPC74 of the HEPC74 and HEPC3 group; 212.1.1 of the 212.1.1 and HC1AM group; and RM2-1 of the RM2-1 and RM11-43 group. Top: the epitopes of AR3-targeted V_H1-69-encoded bnAbs. E2c structures are shown in surface representation and antibody footprints are colored and labelled. Bottom: the position of the HC CDRs in the E2c–Fab structures. E2c domains are shown in surface representation with the front layer and CD81 binding loop as an illustration and colored in gray. The HC CDRs are also illustrated. (b) Conformation of CDRH3 of AR3A, HEPC74, 212.1.1, and RM2-01 from the E2–bnAbs complexes. The intra disulfide bond in the CDRH3 of AR3A and HEPC74 is shown in stick representation.

Figure 4

The conformations of E2 front layer. (a) Superposition of E2c from the crystal structures with AR3C and 212.1.1 indicating conformational changes of the front layer. The two disulfide bonds (C429-C503 and C452-C620) that represent the boundaries of the conformational changes are labeled. (b) Surface representation of E2c from the complex with AR3C (left) and 212.1.1 (right), and E2ecto from the complex with HEPC3 (middle). The conformational changes in the front layer of E2c3 in the 212.1.1 complex expose more of the β-sandwich and back-layer surfaces (colored in red and green respectively, upper panels) that include residues critical for CD81 binding (bottom panels, colored in pink).