Surface-modified measles vaccines encoding oligomeric, prefusion-stabilized SARS-CoV-2 spike glycoproteins boost neutralizing antibody responses to Omicron and historical variants, independent of measles seropositivity

Serum titers of SARSCoV2neutralizing antibodies (nAbs) correlate well with protection from symptomatic COVID19 but decay rapidly in the months following vaccination or infection. In contrast, measlesprotective nAb titers are lifelong after measles vaccination, possibly due to persistence of the liveattenuated virus in lymphoid tissues. We, therefore, sought to generate a live recombinant measles vaccine capable of driving high SARSCoV2 nAb responses. Since previous clinical testing of a live measles vaccine encoding a SARSCoV2 spike glycoprotein resulted in suboptimal antispike antibody titers, our new vectors were designed to encode prefusionstabilized SARSCoV2 spike glycoproteins, trimerized via an inserted peptide domain, and displayed on a dodecahedral miniferritin scaffold. Additionally, to circumvent the blunting of vaccine efficacy by preformed antimeasles antibodies, we extensively modified the measles surface glycoproteins. Comprehensive in vivo mouse testing demonstrated the potent induction of high titer nAbs in measlesimmune mice and confirmed the significant contributions to overall potency afforded by prefusion stabilization, trimerization, and miniferritin display of the SARSCoV2 spike glycoprotein. In animals primed and boosted with a measles virus (MeV) vaccine encoding the ancestral SARSCoV2 spike, hightiter nAb responses against ancestral virus strains were only weakly crossreactive with the Omicron variant. However, in primed animals that were boosted with a MeV vaccine encoding the Omicron BA.1 spike, antibody titers to both ancestral and Omicron strains were robustly elevated, and the passive transfer of serum from these animals protected K18ACE2 mice from infection and morbidity after exposure to BA.1 and WA1/2020 strains. Our results demonstrate that by engineering the antigen, we can develop potent measlesbased vaccine candidates against SARSCoV2. IMPORTANCE Although the liveattenuated measles virus (MeV) is one of the safest and most efficacious human vaccines, a measlesvectored COVID19 vaccine candidate expressing the SARSCoV2 spike failed to elicit neutralizing antibody (nAb) responses in a phase1 clinical trial, especially in measlesimmune individuals. Here, we constructed a comprehensive panel of MeVbased COVID19 vaccine candidates using a MeV with extensive modifications on the envelope glycoproteins (MeVMR). We show that artificial trimerization of the spike is critical for the induction of nAbs and that their magnitude can be significantly augmented when the spike protein is synchronously fused to a dodecahedral scaffold. Furthermore, preexisting measles immunity did not abolish heterologous immunity elicited by our vector. Our results highlight the importance of antigen optimization in the development of spikebased COVID19 vaccines and therapies.