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Herein, we report that a recombinant fusion protein, containing a 457 amino acid SARS-CoV-2 receptor binding domain (RBD, residues 319–541) and a mouse IgG1 Fc domain, could induce highly potent neutralizing antibodies and stimulate humoral and cellular immunity in mice. The antibodies also effectively suppressed SARS-CoV-2 RBD binding to soluble ACE2, indicating that RBD-mFc may be further developed as a safe and effective SARS-CoV-2 vaccine. [ABSTRACT FROM AUTHOR]

Toll-like receptor 4 (TLR4) is a reliable target for the development of vaccine adjuvants. To identify novel TLR4 ligands with improved immunological properties for use as adjuvants for a RBD-hFc based SARS-CoV-2 vaccine, herein, natural E. coli monophosphoryl lipid A (MPLA) and nine of its derivatives were designed and synthesized. Immunological evaluation showed that compounds 1, 3, 5 and 7 exhibited comparative or better adjuvant activity than clinically used Al adjuvants, and are expected to be a promising platform for the development of new adjuvants used for a RBD-hFc based SARS-CoV-2 vaccine. Preliminary structure-activity relationship analysis of the MPLA derivatives showed that the replacement of the functional groups at the C-1, C-4' or C-6' position of E. coli MPLA has an effect on its biological activity. In addition, we found that the combination of MPLA and Al was feasible for immunotherapy and could further enhance immune responses, providing a new direction toward the immunological enhancement of RBD-hFc based SARS-CoV-2 vaccines., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)