From Monovalent to Multivalent Vaccines, the Exploration for Potential Preventive Strategies Against Hand, Foot, and Mouth Disease (HFMD)

Virologica Sinica https://doi.org/10.1007/s12250-020-00294-3 www.virosin.org www.springer.com/12250 (0123456789().,-volV)(0123456789().,-volV) REVIEW From Monovalent to Multivalent Vaccines, the Exploration for Potential Preventive Strategies Against Hand, Foot, and Mouth Disease (HFMD) Xiangchuan He1 • Miaomiao Zhang1 • Chen Zhao1 • Peiyong Zheng2 • Xiaoyan Zhang1 • Jianqing Xu1 Received: 22 April 2020 / Accepted: 25 August 2020 Ó Wuhan Institute of Virology, CAS 2020 Abstract Hand, foot, and mouth disease (HFMD) recently emerged as a global public threat. The licensure of inactivated enterovirus A71 (EV-A71) vaccine was the first step in using a vaccine to control HFMD. New challenges arise from changes in the pathogen spectrum while vaccines directed against other common serotypes are in the preclinical stage. The mission of a broad-spectrum prevention strategy clearly favors multivalent vaccines. The development of multivalent vaccines was attempted via the simple combination of potent monovalent vaccines or the construction of chimeric vaccines comprised of epitopes derived from different virus serotypes. The present review summarizes recent advances in HFMD vaccine development and discusses the next steps toward a safe and effective HFMD vaccine that is capable of establishing a crossprotective antibody response. Keywords Hand, foot, and mouth disease (HFMD)  Inactivated whole virus vaccine  Virus-like particles  Multivalent vaccines  Chimeric vaccines Introduction Human hand-foot-and-mouth disease (HFMD) caused several large outbreaks across the Asian-Pacific region, and it represents a global public health issue. Several viruses were identified as the primary HFMD-related pathogens, and this list includes enterovirus A71 (EV-A71), coxsackievirus A16 (CV-A16), CV-A6 and CV-A10, which all belong to the genus Enterovirus within the Picornaviridae family (Fang and Liu 2018). HFMD frequently occurs in children under five years old, and it is generally characterized by vesicular exanthema with self-limitation. There appears to be a link between the range of clinical & Jianqing Xu & Xiaoyan Zhang 1 Shanghai Public Health Clinical Center and Institutes of Biomedical Science, Shanghai Medical College, Fudan University, Shanghai 201508, China 2 Longhua Hospital, Shanghai University of Traditional Chinese Medicine, Shanghai 200032, China manifestations and serotype differences, with some EVA71 infections resulting in severe complications, including brainstem encephalitis, aseptic meningitis, acute flaccid paralysis, cardiopulmonary failure, or death, but other serotypes generally showing mild symptoms (Lin et al. 2019). Historically, EV-A71 and CV-A16 primarily accounted for the global HFMD outbreaks; however, other serotypes are gradually gaining dominance due to the broad inoculation of and protection by inactivated EV-A71 vaccines. Indeed, CV-A6 displaced EV-A71 and CV-A16 as the predominant serotype in 2013 in Shanghai, and CVA10 has gradually become the dominating HFMD-related enterovirus (Song et al. 2017; Wang J et al. 2018; Bian et al. 2019). Enteroviruses are positive-stranded RNA viruses with a genome size of approximately 7.4 kb, which encodes a single polyprotein of * 2100 amino acids. The polyprotein is divided into three subregions, namely, P1, P2, and P3. The P1 region encodes four structural proteins (VP4– VP2–VP3–VP1), and the P2 and P3 regions encode seven nonstructural proteins (P2–2A, 2B, 2C; P3–3A, 3B, 3C, 3D) (Fig. 1). The four structural proteins assemble to form the basic building block of the virion capsid, namely, a 123 Virologica Sinica Structural proteins Nonstructural proteins P1 P3 P2 IRES 5'UTR VP4 VP2 VP3 VP1 2A 2B 2C 3A 3B 3C 3D 3'UTR Poly A VPg Enterovirus RNA ≈ 7.4 kb Protease: Translation and processing 2Apro 3CD VP0 VP4 VP3 VP2 VP1 2BC 2Apro 2B 2C 3AB 3A VPg 3Cpro 3CD 3Cpro 3Dpro Fig. 1 The structure of enterovirus 71 genome and virion organization. The RNA genome of EV-A71 is approximately 7.4 kb, with an untranslated region (UTR) at the 50 and 30 ends of the genome. The 50 UTR contains an internal ribosomal entry site (IRES) for cap- independent translation. The 50 UTR is bound covalently to VPg (3B), and the 30 UTR includes a poly-A tail. The RNA is translated to a polyprotein that is sequentially cleaved by the viral 2A protease (2Apro), 3CD protease, and 3C protease (3Cpro). protomer. Five protomers come together to form a pentamer, and 12 pentamers plus the viral genome form an icosahedral virion of * 30 nm diameter (Yi et al. 2017). Among the three abovementioned factors, the cellular immune response is the most feasible target for vaccinebased prevention. Host Immune Responses to Natural Infection of HFMD-Related Viruses Critical Epitopes Recognized by Neutralizing Antibodies Humoral immune responses against HFMD-related viruses produce virus-specific neutralizing antibodies, which are generally sufficient to curb virus spreading and makes HFMD a self-limiting disease. However, there were reported cases in infants and young children where severe complications developed despite normal or nearly normal antibody titers compared to patients with mild HFMD, which indicates that other factors contribute to the disease severity (Lim and Poh 2019). The following factors may influence severity: (1) Variation in the IgG composition. The different IgG subclasses that elicited by the viral infection behave differently in virus control. For instance, the IgG1 subclass, and to a lesser extent the IgG2 subclass, primarily mediate the virus-neutralizing activity, while the IgG3 subclass does not (Cao et al. 2013). (2) Variation in cellular immunity. The circulating virus-specific CD8? T cells and CD4? T cells must be effectively engaged for the timely clearance of virus-infected cells and helping the antibody production (Aw-Yong et al. 2019). (3) Genetic variations. There may be intrinsic differences between individuals in countering a viral invasion due to inherited variations in host factors that determine viral susceptibility at the cellular and organismal levels (Yee and Poh 2018). As components of the virion capsid, VP1, VP2 and VP3 are the main targets of human neutralizing antibodies. VP1 contributes to the majority of neutralizing epitopes, and its binding may be used as a valuable assay to assess vaccine potency. VP2 and VP3 proteins harbor fewer neutralizing epitopes compared to VP1, despite structural similarity. Among the common HFMD-related enteroviruses, EV-A71 and CV-A16 show high conservation in capsid proteins, with approximately 80% sequence identity, and their neutralizing epitopes are largely overlapped (Anasir and Poh 2019). The neutralizing epitopes are classified into linear epitopes and conformational epitopes. Most of the linear epitopes are located in the B–C, E–F, and G–H loops, and the C-terminus of VP1, the E–F loops of VP2, and N-terminal region (...truncated)