STING and TLR9 agonists synergistically enhance the immunogenicity of SARS-CoV-2 subunit vaccine

Vaccines that are reliable and efficacious are essential in the fight against the COVID-19 pandemic. In this study, we designed a dual-adjuvant system with two pathogen-associated molecular patterns (PAMPs), MnO_x and CpG. This system can improve the retention of antigens at the injection site, facilitate pro-inflammatory cytokines secretion, further recruit and activate dendritic cells (DCs). As a result, antigens can be delivered to lymph nodes specifically, and adaptive immunity was strengthened. The immunized group showed an enhanced and broadened humoral and cellular immune response in systemic immunity and lung protection when combined with a tandem repeat-linked dimeric antigen version of the SARS-CoV-2 receptor binding domain (RBD_dimer). Remarkably, even with a significant reduction in antigen dosage (three times lower) and a decrease in injection frequencies, our nanovaccine was able to produce the highest neutralizing antibody titers against various mutants. These titers were four-fold higher for the wild-type strain and two-fold higher for both the Beta and Omicron variants in comparison with those elicited by the Alum adjuvant group. In conclusion, our dual-adjuvant formulation presents a promising protein subunit-based candidate vaccine against SARS-CoV-2.