RBD conjugate vaccine with a built-in TLR1/2 agonist is highly immunogenic against SARS-CoV-2 and variants of concern

The coronavirus 2019 (COVID-19) pandemic is causing serious impacts in the world, and safe and effective vaccines and medicines are the best methods to combat the disease. The receptor-binding domain (RBD) of the SARS-CoV-2 spike protein plays a key role in interacting with the angiotensin-converting enzyme 2 (ACE2) receptor, and is regarded as an important target of vaccines. Herein, we constructed the adjuvant-protein conjugate Pam 3 CSK 4 -RBD as a vaccine candidate, in which the N -terminal of the RBD was site-selectively oxidized by transamination and conjugated with the TLR1/2 agonist Pam 3 CSK 4 . This demonstrated that the conjugation of Pam 3 CSK 4 significantly enhanced the anti-RBD antibody response and cellular response. In addition, sera from the Pam 3 CSK 4 -RBD immunized group efficiently inhibited the binding of the RBD to ACE2 and protected cells from SARS-CoV-2 and four variants of concern (alpha, beta, gamma and delta), indicating that this adjuvant strategy could be one of the effective means for protein vaccine development. An RBD-based subunit vaccine with a built-in TLR1/2 agonist induced potent immune responses against SARS-CoV-2 and variants of concern.