Abstract
Exploring potent adjuvants and new vaccine strategies is crucial for the development of protein vaccines. In this work, we synthesized a new TLR4 agonist, structurally simplified lipid A analogue GAP112, as a potent built-in adjuvant to improve the immunogenicity of SARS-CoV-2 spike RBD protein. The new TLR4 agonist GAP112 was site-selectively conjugated on the N-terminus of RBD to construct an adjuvant-protein conjugate vaccine in a liposomal formulation. It is the first time that a TLR4 agonist is site-specifically and quantitatively conjugated to a protein antigen. Compared with an unconjugated mixture of GAP112/RBD, a two-dose immunization of the GAP112-RBD conjugate vaccine strongly activated innate immune cells, elicited a 223-fold increase in RBD-specific antibodies, and markedly enhanced T-cell responses. Antibodies induced by GAP112-RBD also effectively cross-neutralized SARS-CoV-2 variants (Delta/B.1.617.2 and Omicron/B.1.1.529). This conjugate strategy provides an effective method to greatly enhance the immunogenicity of antigen in protein vaccines against SARS-CoV-2 and other diseases.