Identification and characterization of a cross-neutralization epitope of Enterovirus 71

Abstract

Enterovirus 71 (EV71) infections in children manifest as exanthema and are most commonly known as hand-foot-and-mouth disease (HFMD). Because it can cause severe neurological complications like poliomyelitis, EV71 has now emerged as an important neurotropic virus in Asia. EV71 virus has been shown to consist of 3 (A, B and C) genotypes and many subgenotypes. Although EV71 vaccine development has recently yielded promising preclinical results, yet the correlation between the content of antigen(s) in vaccine candidates and the level of protective antibody responses is not established. The neutralization epitope(s) of EV71 antigens could be used as the surrogate biomarker of vaccine potency. Using peptide ELISA, antisera generated from animals immunized with formalin-inactivated EV71 virion vaccine formulated in alum, EV71-specific neutralizing monoclonal antibody (nMAb) and a panel of 153 overlapping synthetic peptides covering the entire sequences of VP1, VP2 and VP3 of EV71, we screened for immunodominant linear neutralization epitope(s). Synthetic peptide VP2-28, corresponding to residues 136-150 of VP2, was found to bind to and inhibit the binding to EV71 of nMAb MAB979 that was found to have cross-neutralizing activity against different genotypes of EV71 virus. In addition, VP2-28 was found to be recognized only by neutralizing antisera generated from rabbits immunized with the formalin-inactivated whole EV71 virion vaccine but not by antisera from immunized mice and rats. During the epitope mapping, a murine EV71 genotype- and strain-specific linear neutralization epitope VP1-43 was identified within residues 211-220 of VP1. Furthermore, based on sequence alignment and structure prediction analysis using poliovirus as the template for molecular modeling, the VP1-43 and VP2-28 epitopes were shown to run in parallel within 0.1 nm and form a rim of the canyon at the junction site of VP1 and VP2 in the viral capsid. In mouse, rat and rabbit immunogenicity studies, a dose-dependent relationship between the number of VP2-28 epitope units measured by a quantitative assay in vaccine preparations and the magnitude of neutralizing titers was demonstrated. VP2-28 has amino acid sequences that are highly conserved among EV71 genotypes, is not affected by formalin-treatment and long-term storage. Thus, VP2-28 could be used as the surrogate biomarker in the potency testing of candidate EV71 vaccines.