Recombinant Receptor-Binding Domains of Multiple Middle East Respiratory Syndrome Coronaviruses (MERS-CoVs) Induce Cross-Neutralizing Antibodies against Divergent Human and Camel MERS-CoVs and Antibody Escape Mutants

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) binds to cellular receptor dipeptidyl peptidase 4 (DPP4) via the spike (S) protein receptor-binding domain (RBD). The RBD contains critical neutralizing epitopes and serves as an important vaccine target. Since RBD mutations occur in different MERS-CoV isolates and antibody escape mutants, cross-neutralization of divergent MERS-CoV strains by RBD-induced antibodies remains unknown. Here, we constructed four recombinant RBD (rRBD) proteins with single or multiple mutations detected in representative human MERS-CoV strains from the 2012, 2013, 2014, and 2015 outbreaks, respectively, and one rRBD protein with multiple changes derived from camel MERS-CoV strains. Like the RBD of prototype EMC2012 (EMC-RBD), all five RBDs maintained good antigenicity and functionality, the ability to bind RBD-specific neutralizing monoclonal antibodies (MAbs) and the DPP4 receptor, and high immunogenicity, able to elicit S-specific antibodies. They induced potent neutralizing antibodies cross-neutralizing 17 MERS pseudoviruses expressing S proteins of representative human and camel MERS-CoV strains identified during the 2012-2015 outbreaks, 5 MAb escape MERSCoV mutants, and 2 live human MERS-CoV strains. We then constructed two RBDs mutated in multiple key residues in the receptor-binding motif (RBM) of RBD and demonstrated their strong cross-reactivity with anti-EMC-RBD antibodies. These RBD mutants with diminished DPP4 binding also led to virus attenuation, suggesting that immunoevasion after RBD immunization is accompanied by loss of viral fitness. Therefore, this study demonstrates that MERS-CoV RBD is an important vaccine target able to induce highly potent and broad-spectrum neutralizing antibodies against infection by divergent circulating human and camel MERS-CoV strains.

Identification of the immunodominant neutralizing regions in the spike glycoprotein of porcine deltacoronavirus

“…Neutralizing antibody titers in mice inoculated with the RBD regions of MERS-CoV and SARS-CoV reached 1:1000 in VN assays after the second immunization (He et al, 2005;Tai et al, 2017). The three recombinant PDCoV proteins we tested generated somewhat lower neutralizing antibody responses, perhaps because the truncated region of the S protein contains non-neutralizing epitopes that compete with the neutralizing epitopes in eliciting antibody responses (Du et al, 2013a).…”

Antibody-mediated protection against MERS-CoV in the murine model

“…Vaccination of the zoonotic host, the dromedary camel, may also effectively curb outbreaks of MERS in endemic regions and limit the risk of viral recombination [34] and significant progress with an orthopox-vectored vaccine for MERS has recently been reported [35]. The potential use of a sub-unit vaccine for MERS in camel vaccination could be aided by varying the sequence of the RBD protein [36] and substituting the human Fc tag with an alternative tag recognised by the camel, to design approaches tailored for animal vaccination, bearing in mind that a single dose vaccine would be ideal in this context. However, future work in our laboratory will also address the value of retaining or removing the Fc tag from the RBD protein for clinical or veterinary iterations of the vaccine.…”

MERS coronavirus outbreak: Implications for emerging viral infections

“…Recently, a high-yield CHO cell line capable of large-scale production of this S1 RBD-Fc fusion product was described, strengthening the possibility of sustainable manufacture and human testing for this potential vaccine antigen (Nyon et al, 2018). Another recent study showed that 5 recombinant RBDs incorporating mutations which arose in different MERS-CoV outbreaks or in camel strains could induce neutralizing antibody responses against several MERS-CoV pseudoviruses (Tai et al, 2017).…”

“…In vivo (Mouse) Humoral response in mice; potential intranasal administration; improved by adjuvant; divergent strains/ escape mutants; CHO cell line (Du et al, 2013;Ma et al, 2014;Nyon et al, 2018;Tai et al, 2017;Zhang et al, 2015Zhang et al, , 2016b T cell and neutralizing antibody responses; entering human clinical trials; potential for veterinary use- (Langenmayer et al, 2018;Volz et al, 2015) ad5 or ad41 adenovirus expressing full-length S…”