The past, present and future of neutralizing antibodies for hepatitis C virus

Abstract

Hepatitis C virus (HCV) is a major cause of liver disease and hepatocellular carcinoma worldwide. HCV establishes a chronic infection in the majority of cases. However, some individuals clear the virus, demonstrating a protective role for the host immune response. Although new all-oral drug combinations may soon replace traditional ribavirin-interferon therapy, the emerging drug cocktails will be expensive and associated with side-effects and resistance, making a global vaccine an urgent priority. T cells are widely accepted to play an essential role in clearing acute HCV infection, whereas the role antibodies play in resolution and disease pathogenesis is less well understood. Recent studies have provided an insight into viral neutralizing determinants and the protective role of antibodies during infection. This review provides a historical perspective of the role neutralizing antibodies play in HCV infection and discusses the therapeutic benefits of antibody-based therapies. This article forms part of a symposium in Antiviral Research on "Hepatitis C: next steps toward global eradication."

Keywords: Epitope; HCV E2 core; Hepatitis C; Neutralization; Transmission.

Fig. 1

Neutralizing activity of anti-HCV antibodies and nanobodies. HCV particles circulate as lipoviral particles (LVPs) in the blood, in complex with low-density lipoproteins. Antibody epitopes are accessible on these LVPs. In an established infection, naïve cells can be infected by extracellular virus or by direct cell–cell transmission between adjacent cells. Antibody-mediated neutralization can occur as particles are released from infected cells (1), preventing HCV entry into naïve hepatocytes. While antibody based therapy appears to inhibit the extracellular route of infection, nanobodies (blue oval) have recently been demonstrated to inhibit the direct transmission of HCV between cells. It remains to be determined if this neutralizing activity is mediated via the blockade of cell-tethered virus (2), or direct inhibition of virus transmission between cell–cell junctions (3).

Fig. 2

Variation in the HCV E2 glycoprotein N-terminus. Diversity of HCV E2N-terminal aa 384–429. (A) An entropy plot was performed to measure variability across viral strains – within HVR1 (aa 384–410), functionally conserved residues are observed at positions 385, 406 and 409. The well-described ‘Epitope I’ conserved region (Zhang et al., 2007) contains epitopes of neutralizing mAbs AP33, 3/11 (Tarr et al.) and HCV1 (Kong et al., 2012a,b). (B) A Kyte-Doolittle hydrophobicity plot reveals the hydrophobic nature of the HVR1 between patient sequences. (C) Residues responsible for cross reactivity of antibodies directed to the HVR1 are highlighted on HCV reference H77 strain. These residues are co-incident with regions possessing hydrophilic amino acids that are likely to be exposed on the surface of the virus particle.

Fig. 3

Antigenic organization of the HCV glycoprotein E2 core structure. The surface of E2 is categorized into three partially-overlapping antigenic regions, that include monoclonal antibody epitopes (labelled on the periphery of the regions) using complementary techniques of binding competition assays and, alanine scanning mutagenesis. Neutralizing antibodies are labelled in red, while non-neutralizing antibodies are labelled in green. When mapped onto the crystal structure of E2 (Kong et al., 2013), these domains highlight the CD81 binding region, possessing conserved neutralization epitopes (blue circle), a β-sheet possessing less well conserved neutralization epitopes (pink circle), and a less organized region that includes epitopes of non-neutralizing mAbs (yellow circle). An additional region containing neutralization epitopes was also revealed, that is partially formed by a helical region between aa 428–442 (burgundy oval). Disulfide bonds stabilizing the structure are highlighted in yellow, and unresolved structures within primary amino acid chain indicated by dashed lines.

Fig. 4

Specific residues defining receptor binding sites and antibody binding residues on the core HCV E2 glycoprotein structure. Important residues are highlighted in red on the reported structure (Kong et al., 2013) (PDB reference 4MWF). (A) Residues involved in the CD81 binding site (G523, W529, G530, D535); (B) binding residues for non-neutralizing mAb AR1B (N540, W549); (C) neutralizing mAb AR3A (S424, G523, P525, G530, D535, N540); (D) neutralizing mAb HC84.22 (N428, W437, L441, F442, Y443); (E) neutralizing mAb 1:7 (G523, W529, G530 D535); and (F) the restricted neutralizing mAb 2/69a (G440, Y443, K446). These images highlight the overlapping nature of the two discrete neutralizing epitope clusters on the core E2 structure, and the lack of neutralizing epitopes in the regions of the protein highlighted in orange, yellow and green.