Immunities Specific to Both of the M Protein Ectodomain and RBD Synergize to Confer Cross-protection against SARS-CoV-2 Infections

The effectiveness of the prototypic SARS-CoV-2 vaccine largely decreased overtime against the emerging virus strains, necessitating the universal vaccine development. The most abundant structural membrane (M) protein is highly conserved in amino acid sequence, which arouses our research interests in developing a universal immunogen based on it. Serological analysis showed that IgG responses specific to its N-terminal peptides can be strongly detected in many serum samples from both convalescent patients and vaccinees receiving inactivated vaccines, indicating the potential existence of human B-cell epitopes in reactive peptides. Microneutralization assays showed that the N-terminal peptide S2M2-30-specific hyperimmune serum was capable of cross-neutralizing the authentic viruses including wild-type HKU-001a, B.1.617.2/Delta, and Omicron subvariant BQ.1.1, and synergized with RBD-specific serum in reinforcing antiviral activities. Strong S2M2-30-specific immunities elicited in hACE2-transgenic mice could effectively inhibit B.1.1.7/Alpha (UK) infections. Our results suggest the potentiality of conserved M peptides as vaccine targets for conferring cross-protections against sarbecoviruses.