Immunogenicity and structures of a rationally designed prefusion MERS-CoV spike antigen

TLDR

MERS‑CoV is a lineage C betacoronavirus that emerged in 2012, causing outbreaks with an estimated 36 % case‑fatality, and its spike glycoprotein mediates receptor recognition, membrane fusion, and is the main target of neutralizing antibodies. The study aims to develop a structure‑based strategy to lock coronavirus spike proteins in the prefusion conformation and demonstrate that the engineered immunogen elicits high neutralizing antibody titers against MERS‑CoV. The authors used structure‑based design to stabilize the prefusion spike and solved high‑resolution cryo‑EM structures of the trimeric MERS‑CoV S ectodomain bound to the stem‑directed neutralizing antibody G4. The structures reveal that G4 targets a glycosylated loop and define four trimer conformations with receptor‑binding domains either tightly packed or rotated into a receptor‑accessible state, suggesting a sequential receptor‑binding mechanism for fusion initiation and providing a foundation for coronavirus vaccine design.

Abstract

Middle East respiratory syndrome coronavirus (MERS-CoV) is a lineage C betacoronavirus that since its emergence in 2012 has caused outbreaks in human populations with case-fatality rates of ∼36%. As in other coronaviruses, the spike (S) glycoprotein of MERS-CoV mediates receptor recognition and membrane fusion and is the primary target of the humoral immune response during infection. Here we use structure-based design to develop a generalizable strategy for retaining coronavirus S proteins in the antigenically optimal prefusion conformation and demonstrate that our engineered immunogen is able to elicit high neutralizing antibody titers against MERS-CoV. We also determined high-resolution structures of the trimeric MERS-CoV S ectodomain in complex with G4, a stem-directed neutralizing antibody. The structures reveal that G4 recognizes a glycosylated loop that is variable among coronaviruses and they define four conformational states of the trimer wherein each receptor-binding domain is either tightly packed at the membrane-distal apex or rotated into a receptor-accessible conformation. Our studies suggest a potential mechanism for fusion initiation through sequential receptor-binding events and provide a foundation for the structure-based design of coronavirus vaccines.