Hepatitis C Virus E1E2 Structure, Diversity, and Implications for Vaccine Development

Abstract

Hepatitis C virus (HCV) is a major medical health burden and the leading cause of chronic liver disease and cancer worldwide. More than 58 million people are chronically infected with HCV, with 1.5 million new infections occurring each year. An effective HCV vaccine is a major public health and medical need as recognized by the World Health Organization. However, due to the high variability of the virus and its ability to escape the immune response, HCV rapidly accumulates mutations, making vaccine development a formidable challenge. An effective vaccine must elicit broadly neutralizing antibodies (bnAbs) in a consistent fashion. After decades of studies from basic research through clinical development, the antigen of choice is considered the E1E2 envelope glycoprotein due to conserved, broadly neutralizing antigenic domains located in the constituent subunits of E1, E2, and the E1E2 heterodimeric complex itself. The challenge has been elicitation of robust humoral and cellular responses leading to broad virus neutralization due to the relatively low immunogenicity of this antigen. In view of this challenge, structure-based vaccine design approaches to stabilize key antigenic domains have been hampered due to the lack of E1E2 atomic-level resolution structures to guide them. Another challenge has been the development of a delivery platform in which a multivalent form of the antigen can be presented in order to elicit a more robust anti-HCV immune response. Recent nanoparticle vaccines are gaining prominence in the field due to their ability to facilitate a controlled multivalent presentation and trafficking to lymph nodes, where they can interact with both the cellular and humoral components of the immune system. This review focuses on recent advances in understanding the E1E2 heterodimeric structure to facilitate a rational design approach and the potential for development of a multivalent nanoparticle-based HCV E1E2 vaccine. Both aspects are considered important in the development of an effective HCV vaccine that can effectively address viral diversity and escape.

Keywords: HCV E1E2; nanoparticles; structure; vaccine.

Figure 1

HCV sequence diversity and representative available structures. The HCV E1E2 genotype and subtype reference sequences were downloaded from the LANL HCV database [61] and are shown as an unrooted phylogenetic tree, with branches and labels colored according to the genotype. The subtypes with currently available experimentally determined E2 or E1E2 glycoprotein structures are labeled, with representative structures (Protein Data Bank code, HCV strain, and protein) given. Multiple-sequence alignment and phylogenetic clustering was performed with the MAFFT web server [62], and the unrooted tree was generated with iTOL [63]. Genotype 7b and 8a reference sequences were not available in the LANL HCV reference sequence set and were downloaded from NCBI (Genbank IDs KX092342, MH590698).

Figure 2

Structure and antigenic regions/domains of the E1E2 heterodimer. Shown is the structure of engineered E1E2 heterodimer ectodomain1, shown in cartoon and semi-transparent surface representation. The E1 and E2 subunits are colored blue and orange, respectively, with mapped antigenic surfaces colored according to the antigenic region (AR) or antigenic domain (AD) targeted: neutralizing face (purple), AR-4 (pink), AR-5 (dark blue), AD-C (teal), and AD-A (light green). The CD81 binding loop and the bridging/base domains are colored yellow and wheat, respectively. N-linked glycans are depicted as gray sticks.

Figure 3

Sub-regions of E1 and E2. Structure of the E1E2 heterodimer shown in cartoon representation and colored according to sub-region (based on PDB ID 8FSJ). The dShown E2 sub-regions include the following: front layer (residues 420–458), variable region 2 (VR2, residues 459–483), β-sandwich core (residues 484–517 and 535–568), CD81 binding loop (CD81 BL, residues 518–534), variable region 3 (VR3, residues 569–579), post-variable 3 region (pVR3, residues 580–595), back layer (BL, residues 596–645), and the bridging (or base) domain (BD, residues 646–701). Shown E1 sub-regions include the following: N-terminal domain (NTD, residues 192–205), E1 core (residues 206–255 and 295–299), and C-terminal loop (CTL, residues 300–314).

Figure 4

E2 and E1E2 structure-based and rational design strategies. Selected reported E2 and E1E2 design approaches are shown on an E1E2 structure (PDB code 8FSJ), with E1, E2, and neutralizing face (antigenic domain B/D, antigenic region 3) colored as in Figure 2. E2 position 445, the site of a previously reported neutralizing face proline design [106], is colored green. E1E2 scaffolding and nanoparticle fusion are shown at the E1 and E2 C-termini, while loop removal is shown at the E2 HVR2 loop.

Figure 5

Multivalent presentation of E1E2 antigens. Coupling to a nanoparticle increases the size and valency of E1E2 antigen presentation. Flexible attachment methods allow for the presentation of genetically diverse sequences as consensus (striped), mosaic (multiple colors), or cocktails of multiple representative genome sequences.