When the MERS CoV outbreak raised global health concerns, we initiated a program to develop a candidate vaccine. Starting from Spike glycoprotein (S) sequences, we developed an immunization strategy consisting of a full-length S DNA prime and a S1 subunit protein boost that elicited high titers of neutralizing antibodies against eight different MERS-CoV strains. Immune sera contained potent neutralizing antibodies targeting the receptor binding domain (RBD), non-RBD portions of S1, and the S2 subunit. From the immunized mice, we produced a panel of hybridomas and produced monoclonal antibodies from which a variety with high neutralizing activity were selected for further characterization. The atomic structure of a monoclonal antibody, D12, in complex with the RBD revealed two distinct mechanisms by which they block binding to the MERS-CoV receptor, DPP4. The full-length S protein was also immunogenic in non-human primates. These initial studies demonstrated that coronovirus spike vaccine immunogens induced a diverse repertoire of neutralizing antibodies, demonstrating an efficient approach to vaccine design that may be applicable to other emerging viruses. Through collaborations, we have solved the structure of the MERS S protein and based on this structure, designed stabilizing mutations which stabilize the S protein in its prefusion conformation. We are currently assessing immunogenicity of the stabilized MERS S protein, and evaluating stabilizing mutations in other coronavirus spike proteins. Additionally, we have continued to characterize the binding and functional characteristics of monoclonal antibodies that are specific for different epitopes on the S protein.
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