Capitalizing on knowledge of hepatitis C virus neutralizing epitopes for rational vaccine design

Abstract

Hepatitis C virus infects nearly 3% of the world's population and is often referred as a silent epidemic. It is a leading cause of liver cirrhosis and hepatocellular carcinoma in endemic countries. Although antiviral drugs are now available, they are not readily accessible to marginalized social groups and developing nations that are disproportionally impacted by HCV. To stop the HCV pandemic, a vaccine is needed. Recent advances in HCV research have provided new opportunities for studying HCV neutralizing antibodies and their subsequent use for rational vaccine design. It is now recognized that neutralizing antibodies to conserved antigenic sites of the virus can cross-neutralize diverse HCV genotypes and protect against infection in vivo. Structural characterization of the neutralizing epitopes has provided valuable information for design of candidate immunogens.

Figure 1. Comparison of antigenic variability of HIV-1 and HCV Env

(a) Phylogeny relationships of envelope glycoprotein amino-acid sequences of HIV (left) and HCV (right) reference viruses inferred by the neighbor-joining method (MEGA4 software). Genotype/subtype reference virus sequences were downloaded from the LANL HIV and HCV sequence database. Scale bar indicates the fraction of amino-acid substitutions. HCV genotypes are generally more diverse than HIV-1 clades. (b) The glycan shield of HIV-1 is more variable than HCV. The number of N-linked glycosylation sites on HIV-1 gp120 ranges from 18-33 (average 25), and from 10-11 sites on HCV E2. Approximately one site is present for every 21 and 33 residues of gp120 and E2, respectively. The relative positions of N-linked glycosylation sites (boxed in green and numbered underneath) and the levels of amino acid insertions within the glycosylation regions (red numbers above green boxes) are marked on the reference HIV-1 gp120 or HCV E2 sequence alignments. Selected sequences are shown in insets to highlight the level of variations around N2-4 of gp120 and N9 of E2. Residues identical to the top sequences and insertions are indicated by ‘.’ and ‘-‘, respectively. Asparagines in the putative glycosylation sites are underlined. Consequently, HIV-1 can change its glycosylation pattern in many more ways than HCV to escape antibody neutralization.

FIGURE 2. Antigenicity of HCV Env

(A) Antigenic regions of E1E2 complex. The viral envelope glycoproteins E1 and E2 form non-covalent heterodimers. A panel of human mAbs isolated from an HCV-immune antibody library by phage display defines 5 distinct antigen regions (ARs) on the E1E2 complex [27,65]. AR1, AR2 and AR3 are present on E2, and AR4 and AR5 on the E1E2 complex. AR3 overlaps with CD81bs on E2. Several mAbs to AR3 and AR4 cross-neutralize diverse HCV genotypes and protect against HCV infection in small animal models. (B) Antigenic surface of E2c. The molecular surface of the E2c structure is colored according to known properties: green, glycan face; grey, occluded face; yellow, non-neutralizing face; and red, neutralizing face. The conserved, accessible neutralizing face contains the E2 CD81bs, AS412 and AS434, and is the prime target for structure-based immunogen design [86,87,94-97,101].