Human enterovirus 71 (EV71), a typical single-stranded, positive-sense RNA virus, belongs to the Enterovirus genus of the Picornaviridae family. In recent years, EV71 has emerged as the most important causative agent of Hand, Foot and Mouse disease (HFMD) affecting mostly young children, especially those younger than 5 years old. The clinical symptoms of EV71 infection include simple exanthema, serious aseptic meningitis, acute flaccid paralysis as well as brainstem encephalitis . Although present in most countries, the largest outbreaks of disease have been seen in the Asia-Pacific region over the past 15 years [2–6], and many areas have experienced cyclical epidemics that occur every 2–3 years [7–10]. EV71 infection has now been recognized as an important global public health issue.
Vaccination probably offers the best option for disease control, but there is no available licensed vaccine against EV71. Several vaccine candidates including formaldehyde-inactivated whole-virus vaccine, live-attenuated vaccine, virus-like particles (VLPs), DNA vaccine and subunit vaccine, have showed promise for clinical use [11–20]. Especially, inactivated EV71 vaccines manufactured in mainland China have undergone phase III clinical trials with ideal efficacy .
The genome of EV71 is about 7.4 kb in length, which first encodes a long polyprotein with a single open reading frame followed by a poly A tract. Then, the polyprotein is divided into three different precursor proteins (P1, P2 and P3). The P1 precursor protein is further cleaved to four structural proteins, including VP0 (the precursor of VP2 and VP4), VP3 and VP1, which make up the capsid; whereas P2 and P3 are cleaved to non-structural proteins that are involved in genome replication and translation . Among the capsid protein, VP1, VP2, and VP3 are external, and well exposed to host immune system. Neutralizing antibodies against EV71 have been demonstrated as the most important factors in limiting the severity of infection . Several linear neutralizing epitopes have been identified within the capsid proteins of EV71. Foo and colleagues have characterized two neutralizing epitopes in VP1, SP55 (amino acids 163–177) and SP70 (amino acids 208–222), both were capable of eliciting neutralizing antibodies and conferred protection against homologous and heterologous EV71 strains in neonatal BALB/c mice . Moreover, monoclonal antibody targeting the linear epitope on VP1 protein, spanning amino acids 215–219, provided full protection against EV71 challenge in vivo [25, 26]. Recently, screening with overlapping synthetic peptides covering the entire capsid protein of EV71 identified SP28 (amino acids 136–150 of VP2) as potential neutralizing epitope . Epitope-based vaccines containing well-defined protective epitopes are supposed to stimulate effective and specific protective immune responses while avoiding potential undesirable side-effect [28, 29]. Various epitope-based vaccine candidates against virus infection have been well demonstrated with ideal immunogenicity and protection [30–32], and several promising pre-clinical and clinical trials for vaccines that involve peptide-based strategies are currently being carried out [33, 34], yet only a few epitope-based EV71 vaccines have been described [24, 35, 36].
In this study, a novel EV71 epitope-based vaccine candidate was designed through tandem connecting the three known EV71 neutralizing epitopes with the Gly-Ser linker and expressed in E.coli. This recombinant multiple tandem linear neutralizing epitopes, named mTLNE, was evidenced to evoke humoral and cellular immune responses and passive transfer with anti-mTLNE sera conferred full protection against lethal EV71 challenge in mice.