Establishment of an animal challenge model as a potency assay for an inactivated Enterovirus Type 71 vaccine

Enterovirus 71 (EV71) belongs to the Enterovirus genus of the Picornaviridae family, and its occurrence in Asia is associated with hand-foot-and-mouth disease (HFMD), leading to death in some cases, in young children. An effective EV71 vaccine is therefore urgently needed. In this study, we established a two-step EV71 vaccine potency model. Intraperitoneal injections in 2-day-old suckling mice were used to establish the LD50 of EV71 B4, B5, C2, C4, and C5 subgenotypes. Only C4 caused hind limb paralysis in mice (LD50: 2.62 ± 0.45). EV71 VP1 protein was identified in the brain tissues at histology. In the second phase of the model, 3-week-old female ICR mice received one primary and two boosting i.p. injections of formalin-inactivated EV71 B4 and C4 vaccine. Immunized serum was neutralized in vitro with EV71 C4 and applied to the murine challenge model. The C4 vaccine-immunized serum exhibited the highest protective titre (ED50 = 114.6), while the B4 immunized serum had the weakest protective titre (ED50 = 34.3). Additionally, human plasma and intravenous immunoglobulin displayed significant protection in the neutralization assay. Our results could facilitate candidate EV71 vaccine immunogenicity and efficacy evaluations, and may help establish reference EV71 antisera in the future.