Thermal stabilization of enterovirus A 71 and production of antigenically stabilized empty capsids

Substitution of Coxsackievirus A16 VP1 BC and EF Loop Altered the Protective Immune Responses in Chimera Enterovirus A71

Hand, foot and mouth disease (HFMD) is a childhood disease caused by enterovirus A71 (EV-A71) and coxsackievirus A16 (CV-A16). Capsid loops are important epitopes for EV-A71 and CV-A16. Seven chimeric EV-A71 (ChiE71) involving VP1 BC (45.5% similarity), DE, EF, GH and HI loops, VP2 EF loop and VP3 GH loop (91.3% similarity) were substituted with corresponding CV-A16 loops. Only ChiE71-1-BC, ChiE71-1-EF, ChiE71-1-GH and ChiE71-3-GH were viable. EV-A71 and CV-A16 antiserum neutralized ChiE71-1-BC and ChiE71-1-EF. Mice immunized with inactivated ChiE71 elicited high IgG, IFN-γ, IL-2, IL-4 and IL-10. Neonatal mice receiving passive transfer of WT EV-A71, ChiE71-1-EF and ChiE71-1-BC immune sera had 100%, 80.0% and no survival, respectively, against lethal challenges with EV-A71, suggesting that the substituted CV-A16 loops disrupted EV-A71 immunogenicity. Passive transfer of CV-A16, ChiE71-1-EF and ChiE71-1-BC immune sera provided 40.0%, 20.0% and 42.9% survival, respectively, against CV-A16. One-day-old neonatal mice immunized with WT EV-A71, ChiE71-1-BC, ChiE71-1-EF and CV-A16 achieved 62.5%, 60.0%, 57.1%, and no survival, respectively, after the EV-A71 challenge. Active immunization using CV-A16 provided full protection while WT EV-A71, ChiE71-1-BC and ChiE71-1-EF immunization showed partial cross-protection in CV-A16 lethal challenge with survival rates of 50.0%, 20.0% and 40%, respectively. Disruption of a capsid loop could affect virus immunogenicity, and future vaccine design should include conservation of the enterovirus capsid loops.

Production of antigenically stable enterovirus A71 virus-like particles in Pichia pastoris as a vaccine candidate

Enterovirus A71 (EVA71) causes widespread disease in young children with occasional fatal consequences. In common with other picornaviruses, both empty capsids (ECs) and infectious virions are produced during the viral lifecycle. While initially antigenically indistinguishable from virions, ECs readily convert to an expanded conformation at moderate temperatures. In the closely related poliovirus, these conformational changes result in loss of antigenic sites required to elicit protective immune responses. Whether this is true for EVA71 remains to be determined and is the subject of this investigation.We previously reported the selection of a thermally resistant EVA71 genogroup B2 population using successive rounds of heating and passage. The mutations found in the structural protein-coding region of the selected population conferred increased thermal stability to both virions and naturally produced ECs. Here, we introduced these mutations into a recombinant expression system to produce stabilized virus-like particles (VLPs) in Pichia pastoris.The stabilized VLPs retain the native virion-like antigenic conformation as determined by reactivity with a specific antibody. Structural studies suggest multiple potential mechanisms of antigenic stabilization, however, unlike poliovirus, both native and expanded EVA71 particles elicited antibodies able to directly neutralize virus in vitro. Therefore, anti-EVA71 neutralizing antibodies are elicited by sites which are not canonically associated with the native conformation, but whether antigenic sites specific to the native conformation provide additional protective responses in vivo remains unclear. VLPs are likely to provide cheaper and safer alternatives for vaccine production and these data show that VLP vaccines are comparable with inactivated virus vaccines at inducing neutralising antibodies.

Production of antigenically stable enterovirus A71 virus-like particles in Pichia pastoris as a vaccine candidate

Enterovirus A71 (EVA71) causes widespread disease in young children with occasional fatal consequences. In common with other picornaviruses, both empty capsids (ECs) and infectious virions are produced during the viral lifecycle. While initially antigenically indistinguishable from virions, ECs readily convert to an expanded conformation at moderate temperatures. In the closely related poliovirus, these conformational changes result in loss of antigenic sites required to elicit protective immune responses. Whether this is true for EVA71 remains to be determined and is the subject of this investigation. We previously reported the selection of a thermally resistant EVA71 genogroup B2 population using successive rounds of heating and passage. The mutations found in the structural protein-coding region of the selected population conferred increased thermal stability to both virions and naturally produced ECs. Here, we introduced these mutations into a recombinant expression system to produce stabilised virus-like particles (VLPs) in Pichia pastoris . The stabilised VLPs retain the native virion-like antigenic conformation as determined by reactivity with a specific antibody. Structural studies suggest multiple potential mechanisms of antigenic stabilisation, however, unlike poliovirus, both native and expanded EVA71 particles elicited antibodies able to directly neutralise virus in vitro . Therefore, the anti-EVA71 neutralising antibodies are elicited by sites which are not canonically associated with the native conformation, but whether antigenic sites specific to the native conformation provide additional protective responses in vivo remains unclear. VLPs are likely to provide cheaper and safer alternatives for vaccine production and these data show that VLP vaccines are comparable with inactivated virus vaccines at inducing neutralising antibodies.

Molecular Determinants of Human Rhinovirus Infection, Assembly, and Conformational Stability at Capsid Protein Interfaces

Human rhinovirus (HRV), one of the most frequent human pathogens, is the major causative agent of common colds. HRVs also cause or exacerbate severe respiratory diseases, such as asthma or chronic obstructive pulmonary disease. Despite the biomedical and socioeconomic importance of this virus, no anti-HRV vaccines or drugs are available yet. Protein-protein interfaces in virus capsids have increasingly been recognized as promising virus-specific targets for the development of antiviral drugs. However, the specific structural elements and residues responsible for the biological functions of these extended capsid regions are largely unknown. In this study, we performed a thorough mutational analysis to determine which particular residues along the capsid interpentamer interfaces are relevant to HRV infection as well as the stage(s) in the viral cycle in which they are involved. The effect on the virion infectivity of the individual mutation to alanine of 32 interfacial residues that, together, removed most of the interpentamer interactions was analyzed. Then, a representative sample that included many of those 32 single mutants were tested for capsid and virion assembly as well as virion conformational stability. The results indicate that most of the interfacial residues, and the interactions they establish, are biologically relevant, largely because of their important roles in virion assembly and/or stability. The HRV interpentamer interface is revealed as an atypical protein-protein interface, in which infectivity-determining residues are distributed at a high density along the entire interface. Implications for a better understanding of the relationship between the molecular structure and function of HRV and the development of novel capsid interface-binding anti-HRV agents are discussed. IMPORTANCE The rising concern about the serious medical and socioeconomic consequences of respiratory infections by HRV has elicited a renewed interest in the development of anti-HRV drugs. The conversion into effective drugs of compounds identified via screening, as well as antiviral drug design, rely on the acquisition of fundamental knowledge about the targeted viral elements and their roles during specific steps of the infectious cycle. The results of this study provide a detailed view on structure-function relationships in a viral capsid protein-protein interface, a promising specific target for antiviral intervention. The high density and scattering of the interfacial residues found to be involved in HRV assembly and/or stability support the possibility that any compound designed to bind any particular site at the interface will inhibit infection by interfering with virion morphogenesis or stabilization of the functional virion conformation.