Structural Basis For The Potent Neutralization Of MERS-CoV By A Human Monoclonal Antibody MERS-4

Middle East respiratory syndrome coronavirus(MERS-CoV)was identified in 2012 and causes severe and fatal acute respiratory disease in humans.Its zoonotic infection in humans is highly lethal,with a fatality rate of 36%.However,no prophylactic and therapeutic agents specifically against MERS-CoV are currently available.The envelop spike(S)glycoprotein of MERS-CoV is responsible for the viral entry,and contains a N-terminal S1 subdomain for receptor DPP4.A potent human RBD-specific nuetralizing monoclonal antibody MERS-4 was isolated by Zhang Linqi lab in School of Medicne.MERS-4 was selected form nonimmune human single-chain variable region fragment(scFv)library displayed on the surface of yeast,using soluble MERS-CoV RBD.They optimiaed the yield of MERS-4 antibody by replacing the CDR regions with other antibody and by molecular directed evolution,and got a mutant MERS-4V2,which was similar binding affinity and nuetralaing activity.The major mechanism of neutralizing antibodies against MERS-CoV is directly competing with the cellular receptor DPP4 for binding to the receptor-binding domain(RBD)of the spike glycoprotein.Here,we report crystal structures of a potent neutralizing monoclonal antibody MERS-4 and its variant MERS-4V2 in complex with the RBD,respectively.In contrast to those directly binding to the RBD interface for DPP4 interaction,MERS-4 and MERS-4V2 approach RBD from the opposite side of the interface and recognize a nearly same epitope consisting of RBD ?5-?6,?6-?7 and ?7-?8 loops.By comparing the atomic structures of RBD in unbound,DPP4-bound,MERS-4-bound and MERS-4V2-bound states,we identified a significant conformational change in the ?5-?6 loop of RBD upon antibody binding.Such change involves folding of the ?5-?6 loop towards a shallow groove on the RBD interface critical for accommodating a short helix of DPP4.Mutagenesis in epitope sequence attenuated RBD binding and decreased neutralizing sensitivity to MERS-4 and MERS-4V2.Taken together,our results demonstrated a unique antibody neutralizing epitope on the RBD and an unusual mechanism of action by inducing a conformational change that is expected to bring steric hindrance between RBD and DPP4.Such unique feature also afforded MERS-4 to synergize with other antibodies and provided valuable addition for the combined use of antibodies against MERS-CoV infection.