Enhancing COVID-19 Vaccine Efficacy: Dual Adjuvant\nStrategies with TLR7/8 Agonists and Glycolipids

The\ncontrolled release of immunostimulatory agents represents a\npromising strategy to enhance vaccine efficacy while minimizing side\neffects. This study aimed to improve the efficacy of the RBD-Fc-based\nCOVID-19 vaccine through combining of an iNKT cell agonist and a TLR7/8\nagonist using covalent conjugation and temporal delivery. We hypothesized\nthat these combinations would yield a more balanced Th1/Th2 immune\nresponse. For covalent conjugation, we employed an uncleavable linker\nand a self-immolative disulfide linker to conjugate α-galactosylceramide\n(αGC) to imidazoquinoline (IMDQ). The αGC-SS-IMDQ-Ac conjugate,\ndesigned with a prodrug strategy for controlled TLR7/8 agonist release,\nelicited a higher IFN-γ/IL-4 T cell response ratio than individual\nadjuvants or their admixture. In the temporal delivery approach, administering\nIMDQ followed by αGC after 2 h resulted in the highest IgG2a/IgG1\nratio, significantly surpassing other groups. A 6 h delay between\nglycolipid and IMDQ injections yielded balanced IgG responses, enhancing\nIgG, IgG1, and IgG2a levels synergistically.