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.