Mutations in the Spike Protein of Middle East Respiratory Syndrome Coronavirus Transmitted in Korea Increase Resistance to Antibody-Mediated Neutralization

Middle East respiratory syndrome coronavirus (MERS-CoV) poses a threat to public health. The virus is endemic in the Middle East but can be transmitted to other countries by travel activity. The introduction of MERS-CoV into the Republic of Korea by an infected traveler resulted in a hospital outbreak of MERS that entailed 186 cases and 38 deaths. The MERS-CoV spike (S) protein binds to the cellular protein DPP4 via its receptor binding domain (RBD) and mediates viral entry into target cells. During the MERS outbreak in Korea, emergence and spread of viral variants that harbored mutations in the RBD, D510G and I529T, was observed. Counterintuitively, these mutations were found to reduce DPP4 binding and viral entry into target cells. In this study, we investigated whether they also exerted proviral effects. We confirm that changes D510G and I529T reduce S protein binding to DPP4 but show that this reduction only translates into diminished viral entry when expression of DPP4 on target cells is low. Neither mutation modulated S protein binding to sialic acids, S protein activation by host cell proteases, or inhibition of S protein-driven entry by interferon-induced transmembrane proteins. In contrast, changes D510G and I529T increased resistance of S protein-driven entry to neutralization by monoclonal antibodies and sera from MERS patients. These findings indicate that MERS-CoV variants with reduced neutralization sensitivity were transmitted during the Korean outbreak and that the responsible mutations were compatible with robust infection of cells expressing high levels of DPP4.<b>IMPORTANCE</b> MERS-CoV has pandemic potential, and it is important to identify mutations in viral proteins that might augment viral spread. In the course of a large hospital outbreak of MERS in the Republic of Korea in 2015, the spread of a viral variant that contained mutations in the viral spike protein was observed. These mutations were found to reduce receptor binding and viral infectivity. However, it remained unclear whether they also exerted proviral effects. We demonstrate that these mutations reduce sensitivity to antibody-mediated neutralization and are compatible with robust infection of target cells expressing large amounts of the viral receptor DPP4.