The Enterovirus 71 Procapsid Binds Neutralizing Antibodies and Rescues Virus Infection In Vitro

Abstract

Enterovirus 71 (EV71) is responsible for seasonal outbreaks of hand, foot, and mouth disease in the Asia-Pacific region. The virus has the capability to cause severe disease and death, especially in young children. Although several vaccines are currently in clinical trials, no vaccines or therapeutics have been approved for use. Previous structural studies have revealed that two antigenically distinct capsid forms are produced in EV71-infected cells: an expanded empty capsid, sometimes called a procapsid, and the infectious virus. Specifically, an immunodominant epitope of EV71 that maps to the virus canyon is structurally different in the procapsid and virus. This structure-function study shows that the procapsid can sequester antibodies, thus enhancing EV71 infection in vitro. The results presented here suggest that, due to conformational differences between the EV71 procapsid and virus, the presence of the procapsid in natural virus infections should be considered in the future design of vaccines or therapeutics. IMPORTANCEIn a picornavirus infection, both an infectious and a noninfectious empty capsid, sometimes referred to as a procapsid, are produced. It was novel to discover that the procapsid form of EV71 was expanded and antigenically distinct from the infectious virus. Previously, it had been supposed that this empty capsid was an off-pathway dead end or at best served for storage of pentameric subunits, which was later shown to be unlikely. It remains unexplained why picornaviruses evolutionarily conserve the wasteful production of so much noninfectious capsid. Here, we demonstrate that the EV71 procapsid has different antigenic properties than the infectious virus. Thus, the procapsid has the capacity to sequester neutralizing antibody and protect the virus, promoting or restoring a successful infection in vitro. This important observation should be considered in the future design and development of vaccines and therapeutics.

Cryo-EM Studies of Virus-Antibody Immune Complexes

“…As another example, the bivalent binding pattern of D5 was characterized in which the two Fab IgG fragments could bind to the GH loops of neighboring VP1 molecules related by twofold symmetry, a finding consistent with the observation that D5 IgG was able to neutralize EV-A71 much more potently than D5 Fab (Ye et al 2016). Contrastingly, the 22A12 binding sites near twofold axes on EV-A71 are further apart and bivalent binding of an antibody cannot occur (Shingler et al 2015). In some cases, Fab binding can even change the local arrangement of the E protein to accommodate more Fab molecules.…”

“…Site 1 is located near the icosahedral fivefold axis of EV-A71 (MA28-7), CV-A6 (1D5)) and EV-D68 (11G1)(Zheng et al 2019). While site 2 maps to the VP1 GH-loop across the twofold axis of EV-A71 (22A12 and D5)(Shingler et al 2015;Ye et al 2016), site 3 is situated near the threefold axis of EV-A71 (E18, E19 and A9)(Plevka et al 2014;Zhu L et al 2018b) and EV-D68 (15C5)(Zheng et al 2019). Site 4 is adjacent to the quasi threefold axis of CV-A10 (2G8)(Zhu R et al 2018).Some clinically relevant viruses in the Flaviviridae family, such as dengue virus (DENV), West Nile virus (WNV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV) and Zika virus (ZIKV)…”

Immunogenicity and performance of an enterovirus 71 virus-like-particle vaccine in nonhuman primates

“…Not too unexpectedly, IgG was not present in the control animal euthanized on day 7 and very low in the other two control animals. The viral particles and procapsids produced in control animals during virus replication may have sequestered the low level of IgG, preventing detection by our assay [44]. It is also possible that the virus challenge dose was too low to induce a robust immune response and significant Ig class switching.…”

Delivery of VP1 siRNA to inhibit the EV71 virus using functionalized silver nanoparticles through ROS-mediated signaling pathways

“…[10][11][12] The genome is translated as a single large polyprotein that is composed of four capsid proteins, VP1 to VP4, and seven nonstructural proteins, 2A, 2B, 2C, 3A, 3B, 3C and 3D. [13][14][15] There is a surface depression, called "canyon", around the 5-fold-related plateaus of both the noninfectious empty particle and the infections mature virion. 16 VP1 proteins are located around the icosahedral vefold axes and form the "northern" rim of the canyon.…”